Clay composition for forming ceramic articles and use of the same

Abstract

A composition for forming ceramic articles is disclosed, where the composition comprises: a finely divided (particulate) fired ceramic material; and an unfired clay; wherein the unfired clay has highe

Description

The present invention relates to a clay composition for forming ceramic items and to ceramic items formed from the composition. The present invention also relates to a method for preparing ceramic articles from the clay composition. The forming of ceramic articles, such as sanitary ware and tableware, from clay is a well-established, much practised technique. Ceramic articles may be formed from clay using a number of different techniques, including moulding which may be employed to form articles on a large manufacturing scale and on a smaller scale. Forming items on a wheel, known in the art as ‘throwing’, is also a well known technique, typically employed for smaller scale production or the forming of one-off items. The raw material for forming ceramic articles in these ways is clay, which is combined with water to the appropriate consistency for forming by the selected technique. One the article has been formed, the clay is allowed to dry, forming items known in the art as ‘greenware’. Thereafter, the greenware item is fired in a kiln to form the ceramic article. The surface of the article may be decorated using one or more glazes, which may be applied before firing and / or after firing. If the glaze is applied to the article after firing, the article is typically fired for a second time. During production of ceramic items, there is a significant amount of waste material. The production of a greenware item can result in a significant amount of clay being wasted. For example, waste clay can be produced during the techniques used to form the greenware item. In addition, greenware items may be rejected, for example due to breakage or being outside the required specifications for the finished article. In many cases, it is possible to recycle this clay, for example mixing the waste clay with fresh clay and reconstituting it into a workable clay material. A greater problem arises with waste ceramic material that is produced after the greenware has been fired. A significant amount of waste ceramic material is produced, for example as a result of the article being damaged during firing, such as cracking, the article being outside the required specifications, such as due to excessive shrinkage during firing, or breakage of the fired ceramic article. There is a need for a technique to employ waste ceramic materials formed in these ways. It is known to include fired clay material into wet clay. The fired clay material, referred to as ‘grog’ is incorporated into the wet clay during production for a variety of reasons, including reduced shrinkage of the article during firing, increased strength of the finished ceramic article, improved thermal properties of the finished ceramic article and to provide surface decoration and / or texture. To form grog, already-fired clay is ground into a granular or powdered form and incorporated in the moist clay. The grog may be sourced from broken ceramic articles, flawed pieces, or clay that has been specifically processed and fired for this purpose. A typical grog composition in the clay ranges from 10 to 30% by weight of the dry clay. Grog particles can vary in size from fine powders to large coarse chunks, depending on the effect be sought in the finished article. There is a need for improved techniques to recycle waste ceramic material formed from firing clay. A technique has now been found for using fired ceramic material to form a clay composition that may be employed to form articles from unfired clay. Advantageously, the fired ceramic material can be used as the major component of the clay composition. In a further advantage, the clay composition may be used to form articles employing currently known techniques, such as those discussed above. According to the present invention, there is provided a composition for forming ceramic articles, the composition comprising: a finely divided fired ceramic material; and an unfired clay; wherein the unfired clay has higher plasticity than the ceramic material; and wherein the finely divided fired ceramic material forms the major portion of the composition. It has been found that fired ceramic material that has been divided to form particles can be recycled and used as the basis for a composition that may be formed and fired to produce a ceramic article, provided that it is combined with an unfired clay having a higher plasticity than the finely divided ceramic material. Sufficient unfired clay is employed to provide the composition with the plasticity required for it to be worked and formed using known techniques as discussed above, for example moulding and throwing. In particular, it has been found that the finely divided fired ceramic material may form the major component of the composition, that is comprise more than 50% by weight of the composition. This in turn allows waste ceramic materials to be recycled and reused in significant quantities, reducing the burden on the practices currently employed to dispose of the ceramic waste. The clay composition of the present invention may be used to form ceramic articles. A wide range of articles may be formed, including but not limited to tiles, kitchenware, sanitary ware and decorative articles, such as vessels and sculptures. It is an advantage of the clay composition that it may be used to form ceramic articles using known and well established techniques, for example by moulding. The clay composition comprises a finely divided fired ceramic material. The fired ceramic material is obtained from greenware or other clay or clay articles that have been fired, so as to form ceramic from the clay. Examples of items from which the fired ceramic material may be recovered include kitchenware, sanitary ware, tiles and decorative items. The fired ceramic material used in the composition is finely divided. The term ‘finely divided’ as used herein is a reference to a particulate material that has been subjected to a process to reduce the material to particles. Suitable processes for producing the finely divided material include crushing, grinding and milling. Such processes and equipment for forming the finely divided material are known in the art and are commercially available. In general, the finely divided ceramic material has a particle size appropriate for the technique to be used to form the ceramic article and the desired structure for the ceramic article. For example, a finely divided ceramic material having a small particle size will result in a product formed from a ceramic material having a fine structure, while a larger particle size will result in a ceramic material having a coarser structure. Again, suitable particle sizes will be known to the person skilled in the art familiar with such techniques. The particle size of the finely divided ceramic material may be indicated by its mesh size, that is the smallest size of mesh through which the particles of the material will pass. The finely divided ceramic material may have a particle size of from mesh 200 (74 microns), preferably from mesh 170 (88 microns), more preferably from mesh 140 (105 microns), still more preferably from mesh 120 (125 microns). The finely divided ceramic material may have a particle size of up to mesh 20 (840 microns), preferably up to mesh 30 (590 microns), more preferably up to mesh 40 (420 microns), still more preferably up to mesh 50 (297 microns). For example, the finely divided ceramic material may have a particle size of from mesh 200 (74 microns) to mesh 20 (840 microns), preferably from mesh 170 (88 microns) to mesh 30 (590 microns), more preferably from mesh 140 (105 microns) to mesh 40 (420 microns), still more preferably from mesh 120 (125 microns) to mesh 50 (297 microns). For many embodiments, the finely divided ceramic material has a particle size of at mesh 60 (250 microns) or a smaller particle size, for example a particle size of from mesh 60 (250 microns) to mesh 140 (105 microns) or smaller. A particle size of mesh 60 has been found to be suitable for many embodiments. The finely divided ceramic material may be formed from any fired ceramic material. The finely divided ceramic material may be formed from earthenware or stoneware, for example. Suitable ceramic materials include those derived from the firing of the following clays: Grogged Stoneware, Buff Stoneware, Crank Stoneware, White Stoneware, Black Stoneware, Pink Stoneware, Red Stoneware, Speckle / flecked Stoneware, Sandstone stoneware, Grey Stoneware, Vulcan Black Stoneware, Special Stoneware, High Fire Terracotta, Oxidising St Thomas, Reduction St Thomas, Parian, Porcelain, Modelling Clay, Architectural Clay, Handbuilding Clay, Pizza Oven Clay, Lavafleck, KGM Body, and Ball Clay. Preferred examples of suitable fired ceramic materials that may be used as the starting material for many embodiments include Porcelain, White Stoneware, Buff, St. Thomas, Terracotta and mixtures of two or more thereof. The finely divided ceramic material may be present in the composition in any suitable amount. It is an advantageous aspect of the composition of the present invention that the finely divided ceramic material may be present as the major component, that is the finely divided ceramic material forms at least 50% by weight of the composition. The finely divided ceramic material may be present in an amount of above 50% by weight, more preferably from 55% by weight, still more preferably from 60% by weight, more preferably still from 65% by weight, especially from 70% by weight, more especially from 75% by weight. The finely divided ceramic material may be present in an amount of from 80% by weight, for example from 85%, from 90% or from 95% by weight. The finely divided ceramic material may be present in an amount of up to 99% by weight, more preferably up to 98% by weight, still more preferably up to 97% by weight, more preferably still up to 96% by weight, especially up to 95% by weight, more especially up to 93% by weight. In a preferred embodiment, the finely divided ceramic material is present in an amount of not less than 70% by weight, more preferably not less than 75% by weight, especially not less than 80% by weight. The finely divided ceramic material may be present in an amount of from 50% to 99% by weight, more preferably from 60% to 98% by weight, still more preferably from 65% to 97% by weight, more preferably still from 70% to 96% by weight, especially from 75% to 95% by weight, more especially from 80% to 93% by weight. The composition of the present invention further comprises an unfired clay. As used herein, the term ‘unfired clay’ is a reference to any clay material that has not been fired to form a ceramic material. The unfired clay material may be fresh or virgin clay. It is an advantage of the present invention that waste unfired clay material may also be used. During the conventional production and manufacture of ceramic articles by firing clay, it is common for a significant amount of unfired clay to be produced as a waste material. For example, waste unfired clay may arise from greenware items that are broken or damaged or otherwise do not meet the required specifications. Waste unfired clay may also arise, for example, as part of the production process, for example when clay is trimmed from moulds. The unfired clay is preferably present in the composition in a finely divided or particulate form. In general, the unfired clay has a particle size appropriate for the technique to be used to form the ceramic article and the desired structure for the ceramic article. Suitable particle sizes will be known to the person skilled in the art familiar with such techniques. The unfired clay is employed in a finely divided form, that is as a particulate material. The particle size of the unfired clay material may be indicated by its mesh size, that is the smallest size of mesh through which the particles of the material will pass. The unfired clay may have a particle size of from mesh 200 (74 microns), preferably from mesh 170 (88 microns), more preferably from mesh 140 (105 microns), still more preferably from mesh 120 (125 microns). The unfired clay may have a particle size of up to mesh 20 (840 microns), preferably up to mesh 30 (590 microns), more preferably up to mesh 40 (420 microns), still more preferably up to mesh 50 (297 microns). For example, the unfired clay may have a particle size of from mesh 200 (74 microns) to mesh 20 (840 microns), preferably from mesh 170 (88 microns) to mesh 30 (590 microns), more preferably from mesh 140 (105 microns) to mesh 40 (420 microns), still more preferably from mesh 120 (125 microns) to mesh 50 (297 microns). For many embodiments, the unfired clay has a particle size of at mesh 60 (250 microns) or a smaller particle size, for example a particle size of from mesh 60 (250 microns) to mesh 140 (105 microns) or smaller. A particle size of mesh 60 has been found to be suitable for many embodiments. The unfired clay may be present in the composition in any suitable amount. The unfired clay is employed in an amount sufficient to provide the composition with the required degree of plasticity for it to be worked using the selected technique to form a clay article for firing to produce the ceramic article. As noted above, such techniques include placing the composition in moulds and throwing the composition on a wheel. The unfired clay is preferably present as a minor component of the composition, that is the unfired clay forms less than 50% by weight of the composition. The unfired clay may be present in an amount of from 1% by weight, more preferably from 2% by weight, still more preferably from 3% by weight, more preferably still from 4% by weight. The unfired clay may be present in an amount of up to 50% by weight, for example up to 45% by weight, more preferably up to 40% by weight, still more preferably up to 35% by weight, more preferably still up to 30% by weight, especially up to 25% by weight, more especially up to 20% by weight. In one preferred embodiment, the unfired clay is present in an amount of less than 20% by weight, preferably less than 18% by weight, more preferably 16% by weight or less. The unfired clay may be present in an amount of from 1% to 50% by weight, for example from 1.5% to 45% by weight, more preferably from 2% to 40% by weight, still more preferably from 2.5% to 35% by weight, more preferably still from 3% to 30% by weight, especially from 3.5% to 25% by weight, more especially 4% to 20% by weight. The amount of the unfired clay that may be present in the composition may be determined by the properties of the clay being used. For example, clays having a high rate of shrinkage during firing may only be used in lower amounts, if major cracking of the article during firing is to be avoided. The unfired clay may be any clay material that has a higher plasticity than the finely divided ceramic material. In this respect, ‘plasticity’ is the property of the clay that allows it to change shape without rupturing when force is applied to it. The plasticity of the clay material is a reference to the plastic properties of the clay particles when combined with water, to form a workable clay material, as is well known in the art. In the present invention, the finely divided ceramic material does not form a plastic material when combined with water. This is in contrast to the unfired clay, as noted above. Plasticity of clay used in pottery to form ceramic articles cannot be measured by any scientifically repeatable test. Therefore its measurement in absolute terms is subjective. However, in the present invention, the unfired clay material is selected to have a plasticity, that is the ability to change shape without rupturing, that is greater than the plasticity of the finely divided ceramic material under the same conditions. In industrial scale production processes, the plasticity of a clay is often gauged by the way a clay behaves in forming machines, how it dries and by its stickiness. Technicians look at particle size, shape, surface area information on data sheets to extrapolate plasticity. However, potters find that a more practical way to compare the plasticities of two clays is by throwing samples of the clays on a potters wheel under the same conditions, such as water content of the clay. In general, plastic clays are more responsive, large thin pieces of plastic clays can be made, and made faster, wet pieces of the clays can be moved without excessive deformation and plastic clays centre more easily during throwing on a wheel. In contrast, less- or non-plastic clays tend to split at their edges during wedging and rolling, generate a lot of slip, are more difficult to centre during throwing, are more flabby and unresponsive and require more refining work in the latter stages of the process of forming the clay article for firing. A procedure well known in the art for assessing the plasticity of a particulate material is as follows: 1. Combine the particulate material with water to form a mouldable (workable) material. 2. Form the material into a small cylinder having a diameter of 2cm and a length of 10 cm. 3. Bend the cylinder over on itself to form a coil. Bending the cylinder into a coil without any cracking of the material along the bend indicates that the material has a plasiticity sufficient for use in forming greenware and clay articles. In contrast, the occurrence of cracking along the bend indicates the material has a low plasticity and is not suitable for forming into greenware and clay articles. Clay materials lose their plasticity when fired in a kiln. Accordingly, applying this test to a mouldable material formed from the finely divided ceramic material and water will result in cracking along the bend of the cylinder, indicating a very low or zero plasticity. Applying this test to a mouldable material formed from the finely divided unfired clay and water will result in no cracking along the bend of the cylinder, indicating a material with a higher plasticity. Examples of suitable clay materials that may be used are the same materials as the starting materials used to form the finely divided ceramic material. In one embodiment, the clay material is the same as the clay material from which the finely divided ceramic material is formed. Suitable clay materials include Grogged Stoneware, Buff Stoneware, Crank Stoneware, White Stoneware, Black Stoneware, Pink Stoneware, Red Stoneware, Speckle / flecked Stoneware, Sandstone stoneware, Grey Stoneware, Vulcan Black Stoneware, Special Stoneware, High Fire Terracotta, Oxidising St Thomas, Reduction St Thomas, Parian, Porcelain, Modelling Clay, Architectural Clay, Handbuilding Clay, Pizza Oven Clay, Lavafleck, KGM Body, and Ball Clay. Preferred examples of suitable fired ceramic materials that may be used as the starting material for many embodiments include Porcelain, White Stoneware, Buff, St. Thomas, Terracotta and mixtures of two or more thereof. Preferred clays for many embodiments include Porcelain, White Stoneware, Buff, St. Thomas, Terracotta and mixtures of two or more thereof. These clay materials may be fresh or virgin material and / or waste clay material recovered during the production or manufacture of ceramic articles. Before being fired, these clay materials have a higher plasticity than the finely divided ceramic material, under the same conditions. As noted above, the unfired clay material may comprise an earthenware clay. These have a firing temperature, that is the temperature at which they are fired in a kiln, of from 1000 to 1140°C. In one preferred embodiment, the unfired clay material has a firing temperature that allows the composition to fired at a temperature above 1200°C. This is particularly preferred when the composition comprises a high silica-content material, such as organic ash, as discussed in more detail below. Organic ash must be fired to a temperature above 1200°C in order for the components of the ash to fuse and form a glass. Clays that have a lower firing temperature and that would melt at temperatures of 1200°C or above are preferably avoided when a high silica material, such as organic ash, is employed in the composition. In one preferred embodiment, the unfired clay comprises a montmorillonite clay. Montmorillonite clays are characterised by having a high plasticity, making them particularly suitable for use in compositions of the present invention. According to a further aspect of the present invention, there is provided a clay composition for forming ceramic articles, the composition comprising: a finely divided fired ceramic material; and an unfired clay material comprising a montmorillonite clay. The unfired clay material may comprise any montmorillonite clay or mixture thereof. Bentonite is a preferred montmorillonite clay for use in the present invention. The unfired clay may consist essentially of a montmorillonite clay. Alternatively, the unfired clay material may comprise montmorillonite clay in combination with one or more other unfired clays, for example the clay materials discussed hereinbefore. Montmorillonite clays are known for a variety of uses, for example as an additive for soil and as a desiccant for removing moisture from gasses. As a result, montmorillonite clays are commercially available. Montmorillonite clays have a tendency to undergo a high degree of shrinkage when fired. This is particularly the case with bentonite. This shrinkage can lead to cracking of the clay article during firing. Therefore, the amount of montmorillonite clay is preferably selected to avoid shrinkage and cracking of the article during firing. As a result, the montmorillonite clay is preferably present in the composition in an amount of up to 15% by weight, for example up to 12% by weight, more preferably up to 10% by weight, still more preferably up to 9% by weight, more preferably still up to 8% by weight, especially up to 7.5% by weight, more especially up to 7% by weight, in particular up to 6.5% by weight, more particularly up to 6% by weight. The montmorillonite clay may be present in an amount of from 1% by weight, preferably from 2% by weight, more preferably from 3% by weight, still more preferably from 4% by weight. An amount of from 3 to 8% by weight is particularly suitable for many embodiments, preferably from 4 to 7% by weight, more preferably from 4.5 to 6% by weight. The unfired clay material may consist essentially of a montmorillonite clay material or may comprise montmorillonite clay in combination with one or more other unfired clays, for example the clay materials mentioned hereinbefore. In one embodiment, the unfired clay material comprises a mixture of a first unfired clay consisting of montmorillonite and a second unfired clay consisting of one or more other unfired clays, such as the clays described hereinbefore, for example Porcelain, Buff, St. Thomas or Terracotta. The first and second unfired clays may be combined in any suitable amounts and ratios. The first unfired clay and the second unfired clay may be present in a weight ratio of from 5:1 to 1:5, preferably from 4:1 to 1:4, more preferably from 3:1 to 1:3, still more preferably from 2:1 to 1:2. In many embodiments it is preferred that the second unfired clay is present in weight excess to the first unfired clay. Therefore, for many embodiments, it is preferred that the weight ratio of the first unfired clay and the second unfired clay is from 1:0.2, more preferably from 1:0.3, still more preferably from 1:0.4, more preferably still from 1:0.5. The weight ratio of the first unfired clay to the second unfired clay may be up to 1:0.99, preferably up to 1:0.95, more preferably up to 1:0.9, still more preferably up to 1:0.85, more preferably still up to 1:0.8, especially up to 1:0.75, more especially up to 1:0.7, still more especially up to 1:0.65. In one preferred embodiment, the weight ratio of the first unfired clay and the second unfired clay is from 1:0.4 to 1:0.8, preferably from 1:0.5 to 1:0.7, still more preferably from 1:0.55 to 1:0.65, with a weight ratio of about 1:0.6 being preferred for many embodiments. The composition may further comprise a component having a high silica content. During firing of the composition, the silica fuses and forms a glass-like material, which acts to bind the particles of the ceramic material together, thereby improving the strength of the finished ceramic article. One preferred component having a high silica content is organic ash obtained by burning organic materials, such as plants and plant parts, in particular the wood of trees. Wood ash is a particularly preferred organic ash for use in the present invention. Organic ash, including wood ash, may be characterised as having the following general composition: Silica 30 to 70% by weight; Alumina 10 to 30% by weight; Calcium up to 30% by weight; Potassium up to 15% by weight; Phosphorous up to 13% by weight; Magnesium oxide up to 10% by weight; Iron up to 5% by weight; Sodium up to 5% by weight; and Sulphur up to 5% by weight. The composition of wood ashes has been analysed by Orogbade, B.O., et al., ‘Chemical and physical characteristics of blended cements produced from softwood ash’, Journal of Civil and Environmental Studies, March 2020, indicate that softwood ash contains silica in an amount of about 62% by weight. The same authors indicate that hardwood ash can have a slightly higher silica content of about 65% by weight (Raheem, A. A. et al., ‘Characteristics of Blended Cements Produced from Selected Hardwood Ashes’, Journal of Engineering and Technology, 3(1), pages 61 to 66). Wood ash is known for use in creating ceramic articles, in particular for inclusion in glazes to provide aesthetically pleasing patterns on the surface of the article after firing. However, in the present invention, organic ash, for example wood ash, is being included in the clay forming the body of the article before firing, in order to provide the body of the clay with a higher silica content, as discussed above. The amount of organic ash included in the composition will depend upon such factors as the silica content of the ash and the other components present in the composition. The organic ash may be present in an amount of from 0.5% by weight, more preferably from 1% by weight, still more preferably from 1.5% by weight, more preferably still from 2% by weight, especially from 2.5% by weight, more especially from 3% by weight, still more especially from 3.5% by weight. The organic ash may be present in an amount of up to 15% by weight, for example up to 12% by weight, more preferably up to 10% by weight, still more preferably up to 8% by weight, more preferably still up to 7% by weight, especially up to 6% by weight, more especially up to 5% by weight. In many embodiments, organic ash in an amount of from 1 to 10% by weight has been found to be particularly suitable, preferably from 2 to 8% by weight, more preferably from 3 to 7% by weight, still more preferably 3.5 to 6% by weight, for example from 3.5% to 5% by weight. In some preferred embodiments, the organic ash is present in the composition in an amount of up to 8% by weight, more preferably up to 7% by weight, still more preferably up to 6% by weight. The organic ash is preferably employed in a finely divided form, that is as a particulate material. The particle size of the organic ash may be indicated by its mesh size, that is the smallest size of mesh through which the particles of the material will pass. The organic ash may have a particle size of from mesh 200 (74 microns), preferably from mesh 170 (88 microns), more preferably from mesh 140 (105 microns), still more preferably from mesh 120 (125 microns). The organic ash may have a particle size of up to mesh 20 (840 microns), preferably up to mesh 30 (590 microns), more preferably up to mesh 40 (420 microns), still more preferably up to mesh 50 (297 microns). For example, the organic ash may have a particle size of from mesh 200 (74 microns) to mesh 20 (840 microns), preferably from mesh 170 (88 microns) to mesh 30 (590 microns), more preferably from mesh 140 (105 microns) to mesh 40 (420 microns), still more preferably from mesh 120 (125 microns) to mesh 50 (297 microns). For many embodiments, the organic ash has a particle size of at mesh 60 (250 microns) or a smaller particle size, for example a particle size of from mesh 60 (250 microns) to mesh 140 (105 microns) or smaller. A particle size of mesh 60 has been found to be suitable for many embodiments. As noted above, it has been found that a finely divided ceramic material may be combined with an organic ash, such as wood ash, to provide a clay composition that may be processed and worked using conventional techniques, such as moulding. Therefore, according to a further aspect of the present invention, there is provided a composition for forming ceramic articles, the composition comprising: a finely divided fired ceramic material; and an organic ash. The finely divided ceramic material may be present in the composition with the organic ash in an amount of from 1 above 50% by weight, more preferably from 55% by weight, still more preferably from 60% by weight, more preferably still from 65% by weight, especially from 70% by weight, more especially from 75% by weight. The finely divided ceramic material may be present in an amount of from 80% by weight, for example from 85%, from 90% or from 95% by weight. The finely divided ceramic material may be present in an amount of up to 99% by weight, more preferably up to 98% by weight, still more preferably up to 97% by weight, more preferably still up to 96% by weight, especially up to 95% by weight, more especially up to 93% by weight. The organic ash may be used in combination with an unfired clay, as described hereinbefore. Therefore, the present invention also provides a composition for forming ceramic articles, the composition comprising: a finely divided fired ceramic material; an organic ash; and an unfired clay. The finely divided ceramic material may be present in the composition with the unfired clay and the organic ash in an amount of from 1% by weight, preferably from 2% by weight, more preferably from 4% by weight, still more preferably from 5% by weight. The finely divided ceramic material may be used as the major component, that is forming more than 50% by weight of the composition. The finely divided ceramic material may be used in an amount above 50% by weight, more preferably from 55% by weight, still more preferably from 60% by weight, more preferably still from 65% by weight, especially from 70% by weight, more especially from 75% by weight. The finely divided ceramic material may be present in an amount of from 80% by weight, for example from 85%, from 90% or from 95% by weight. The finely divided ceramic material may be present in an amount of up to 99% by weight, more preferably up to 98% by weight, still more preferably up to 97% by weight, more preferably still up to 96% by weight, especially up to 95% by weight, more especially up to 93% by weight. The ceramic material is employed in the composition in a finely divided form, that is as a particulate material. The particle size of the ceramic material is as described hereinbefore. Any suitable organic ash may be employed to provide the required silica content of the composition. Preferred organic ashes are those formed from wood, including ash obtained by burning hardwoods, softwoods or mixtures thereof. The amount of organic ash included in the composition will depend upon such factors as the silica content of the ash and the other components present in the composition. The organic ash may be present in an amount of from 0.5% by weight, more preferably from 1% by weight, still more preferably from 1.5% by weight, more preferably still from 2% by weight, especially from 2.5% by weight, more especially from 3% by weight, still more especially from 3.5% by weight. The organic ash may be present in an amount of up to 15% by weight, for example up to 12% by weight, more preferably up to 10% by weight, still more preferably up to 8% by weight, more preferably still up to 7% by weight, especially up to 6% by weight, more especially up to 5% by weight. In many embodiments, organic ash in an amount of from 1 to 10% by weight has been found to be particularly suitable, preferably from 2 to 8% by weight, more preferably from 3 to 7% by weight, still more preferably 3.5 to 6% by weight, for example from 3.5% to 5% by weight. In some preferred embodiments, the organic ash is present in the composition in an amount of up to 8% by weight, more preferably up to 7% by weight, still more preferably up to 6% by weight. The organic ash is preferably employed in a finely divided form, that is as a particulate material. The particle size of the organic ash may be indicated by its mesh size, that is the smallest size of mesh through which the particles of the material will pass. The organic ash may have a particle size as hereinbefore described. If an unfired clay is included in the composition, the unfired clay preferably has higher plasticity than the ceramic material, as described hereinbefore. The unfired clay may be present in the composition in any suitable amount. If employed, the unfired clay is employed in an amount sufficient to provide the composition with the required degree of plasticity for it to be worked using the selected technique to form a clay article for firing to produce the ceramic article. As noted above, such techniques include placing the composition in moulds and throwing the composition on a wheel. The unfired clay may be present in the composition in an amount of up to 98% by weight, preferably up to 95% by weight, more preferably up to 95% by weight. The unfired clay is preferably present as a minor component of the composition, that is the unfired clay forms less than 50% by weight of the composition. The unfired clay may be present in an amount of from 1% by weight, more preferably from 2% by weight, still more preferably from 3% by weight, more preferably still from 4% by weight. The unfired clay may be present in an amount of up to 50% by weight, for example up to 45% by weight, more preferably up to 40% by weight, still more preferably up to 35% by weight, more preferably still up to 30% by weight, especially up to 25% by weight, more especially up to 20% by weight. The unfired clay material is employed in the composition in a finely divided form, that is as a particulate material. The particle size of the ceramic material is as described hereinbefore. In one embodiment, the composition of the present invention has the following composition: 88% by weight unfired clay material, for example Porcelain, White Stoneware, Buff, St Thomas or Terracotta; 4.65% by weight wood ash; and 6.98% by weight fired ceramic material, for example Porcelain, White Stoneware, Buff, St Thomas or Terracotta. The composition may be used by first being combined with 30% by volume of water to form a workable clay composition. In a further embodiment, the composition of the present invention has the following composition: 81 to 96% by weight fired ceramic material, for example Porcelain, Buff, St Thomas or Terracotta; 0 to 9.5% by weight unfired clay, for example Porcelain, Buff, St Thomas or Terracotta; 0 to 5.66% by weight bentonite; and 0 to 3.77% by weight wood ash. The composition may be used by first being combined with 30% by volume of water to form a workable clay composition. The aforementioned compositions of the present invention are generally dry compositions, comprising the components in finely divided or particulate form, with little to no water. In use, the dry compositions of the present invention are combined with water to form a workable clay composition. As used herein, the term ‘workable clay’ is a reference to a clay material containing water, that may be worked using known techniques, such as moulding and throwing on a wheel, to form a clay article or greenware. The amount of water employed is determined by the manner in which the clay composition is to be used and the techniques employed to form a clay article or greenware and the properties of the clay required for such techniques. The appropriate amount of water will be readily determined by the person skilled in the art. Accordingly, the present invention further provides a workable clay composition comprising: a clay composition as hereinbefore described; and water. In general, water may be employed in an amount of from 5% by volume of the total composition, preferably from 10% by volume, more preferably from 15% by volume, still more preferably from 20% by volume, more preferably still from 25% by volume. Water may be employed in an amount of up to 70% by volume, preferably up to 65% by volume, more preferably up to 60% by volume, still more preferably up to 55% by volume. Water may be employed in an amount of from 20 to 70% by volume, preferably from 25 to 65% by volume. In many preferred embodiments, water is employed in an amount of from 20 to 60% by volume, more preferably from 25 to 55% by volume, still more preferably from 30 to 50% by volume. In embodiments where the composition consists essentially of a finely divided ceramic material and an organic ash, with little or no unfired clay, higher amounts of water may be required to provide a workable material having the appropriate properties. In this case, water may be employed in an amount of from 30 to 80% by volume, more preferably from 35 to 75% by volume, still more preferably from 40 to 70% by volume. In use, as noted above, the dry composition comprising the finely divided, particulate components is combined with water. This forms a workable clay composition, that may be handled and processed using known techniques for forming clay articles, which in turn are fired to form ceramic articles. The dry components and water may be combined using any suitable apparatus and techniques, which are known in the art to the skilled person. In a further aspect, the present invention provides a method for forming a workable clay material for use in forming ceramic articles after firing, the method comprising: providing a dry composition of finely divided components, as described hereinbefore; combining the dry composition with water; and mixing the dry composition and water to form the workable clay. Once the dry components and water have been combined, the resulting composition is preferably treated to remove air retained in the body of the composition. Suitable apparatus and techniques for removing air from the composition are known in the art and familiar to the skilled person. In one preferred embodiment, the dry components and water are mixed and air removed using extrusion, for example using a pugmill, again as known in the art. As described hereinbefore, the workable clay material may be used to form clay articles intended for firing. Techniques for working the clay material include shaping by hand, moulding and throwing on a wheel. It is an advantage of the present invention that the workable clay materials may be employed on a small scale, to produce clay items in small numbers or single items, as well as on a large industrial scale to form ceramic articles. Techniques for employing the workable clay material on both small scales and larger scales are known in the art and familiar to the skilled person. In a further aspect, the present invention provides a clay article for firing, the clay article being formed from a workable clay composition as hereinbefore described. Once the workable clay material has been formed into a clay article or greenware, the article is dried, as known in the art, to form a dried clay article or greenware. The dried article or greenware is then fired. Accordingly, the present invention further provides a method for preparing a ceramic article comprising firing a clay article as hereinbefore described. The dried clay article may be fired using conventional firing techniques. In conventional firing, the dried clay or greenware article is fired twice. First, the article is fired at a typical temperature of about 1000°C to convert the clay material to a bisque. The bisque firing fires the clay to a hardened, ceramic state, typically strong enough to withstand glazing. A glaze may then be applied to the surfaces of the article, after which the glazed article is fired a second time, typically at a temperature of from 1040°C to 1300°C, to produce the finished ceramic article. It is known in the art to omit the bisque firing and to glaze the dried clay or greenware article at the pre-fired stage. However, this can result in faults in the glaze. This method is known in the art as ‘raw-glazing’. It is an advantage of the present invention that the compositions allow for raw-glazing of the unfired clay articles or greenware with a significant reduction in glaze faults. As a result, the present invention may produce ceramic articles of a high quality, with few to no faults in the glazing using a single firing. This in turn significantly reduces the energy consumed in firing, for example in some embodiments by up to about 50% (typically indicated as being 47% in the art). The compositions of the present invention also allow the clay articles to ‘selfglazing’. In this embodiment, the clay article is partially dried, so as to have a leather hardness, that is dry to the touch and firm, but slightly malleable. The surface of the partially dried clay article is treated by applying a mixture of water and organic ash, present in the water in an amount, for example of from 5 to 15% by weight. The thus treated clay article is then fired in a kiln. A further layer of glaze may be added to the fired article and the article fired for a second time, for example to ensure that the finished ceramic article is food-safe. Alternatively, the layer of glaze to render the fired article food-safe may be applied to the article before firing (so-called raw glazing). In a still further aspect, the present invention provides a ceramic article formed by drying and firing a clay article formed from a workable clay composition as hereinbefore described. Still further, the present invention provides a method for forming a ceramic article, the method comprising: firing a dried clay article as hereinbefore described. Embodiments of the present invention will now be described, by way of example only. In the following examples, percentages are weight percent, unless otherwise indicated. The ceramic articles produced in the experiments described in the following examples are shown in the accompanying figures, in which: Figure 1 is an image of a plate formed from the composition of Example 1 using porcelain clay as the waste ceramic; Figure 2 is an image of a plate formed from the composition of Example 2 using buff clay as the waste ceramic; Figure 3 is an image of a plate formed from the composition of Example 3 using terracotta clay as the waste ceramic; Figure 4 is an image of a plate formed from the composition of Example 4 using porcelain clay; Figure 5 is an image of a tile formed from the composition of Example 5 using fired terracotta ceramic material; Figure 6 is an image of a tile formed from the composition of Example 6 using fired porcelain ceramic material; and Figure 7 is an image of a brick formed from the composition of Example 7 using fired buff stoneware ceramic material. EXAMPLES Example 1 A clay composition for forming ceramic articles was prepared from the following components: Porcelain Ceramic Waste 393g Porcelain Clay 46g Bentonite 27g Organic Ash 18g Water 145g The ceramic waste was prepared by grinding fired porcelain clay in a ball mill to form a powder. The ground material was sieved using a 60 mesh sieve to provide a powder. The clay employed was a waste porcelain clay, that is dried, unfired porcelain clay. The clay was ground in a ball mill to form a powder. The ground material was sieved using a 60 mesh sieve to provide a powder. Bentonite was ground in a ball mill and sieved using a 60 mesh sieve to form a powder. The organic ash employed was prepared by burning coal formed from olive stones. The organic ash was ground in a ball mill and the ground material sieved using a 60 mesh sieve to form a powder. The dry powdered ceramic waste, clay, bentonite clay and wood ash were combined and mixed with the water. The amount of water was equivalent to an amount of 30% volume of the mixture. The resulting mixture was transferred to a de-airing pugmill and processed to fully mix the components and remove air from the composition to form a workable clay material. The resulting material was rolled between two non-stick mats using a slab-roller in conventional manner and then transferred to a non-stick mould to form a plate. The material in the mould was trimmed and smoothed in conventional manner and allowed to dry. Once firm enough to hold its shape and dry to the touch, the plate was removed from the mould and placed on a wooden board to fully dry. The dried plate was glazed using a ceramic spray-gun in a booth and allowed to dry. The dried, glazed plate was fired in a kiln at a temperature of 1220°C in known manner. The resulting plate was of a high quality, having a speckled white appearance. The plate is shown in Figure 1. Example 2 A clay composition for forming ceramic articles was prepared from the following components: Buff Ceramic Waste 393g Buff Clay Bentonite Organic Ash 46g 27g 18g The ceramic waste was prepared by grinding fired buff clay in a ball mill to form a powder. The ground material was sieved using a 60 mesh sieve to provide a powder. The clay employed was a waste buff clay, that is dried, unfired buff clay. The clay was ground in a ball mill to form a powder. The ground material was sieved using a 60 mesh sieve to provide a powder. Bentonite was ground in a ball mill and sieved using a 60 mesh sieve to form a powder. The organic ash employed was prepared by burning coal formed from olive stones. The organic ash was ground in a ball mill and the ground material sieved using a 60 mesh sieve to form a powder. The components were used to prepare a plate by moulding using the general method of Example 1 above. The resulting plate was of a high quality, having a speckled beige appearance. The plate is shown in Figure 2. Example 3 A clay composition for forming ceramic articles was prepared from the following components: Terracotta Ceramic Waste 393g Terracotta Clay 46g Bentonite 27g Organic Ash 18g The ceramic waste was prepared by grinding fired terracotta clay in a ball mill to form a powder. The ground material was sieved using a 60 mesh sieve to provide a powder. The clay employed was a waste terracotta clay, that is dried, unfired terracotta clay. The clay was ground in a ball mill to form a powder. The ground material was sieved using a 60 mesh sieve to provide a powder. Bentonite was ground in a ball mill and sieved using a 60 mesh sieve to form a powder. The organic ash employed was prepared by burning coal formed from olive stones. The organic ash was ground in a ball mill and the ground material sieved using a 60 mesh sieve to form a powder. The components were used to prepare a plate by moulding using the general method of Example 1 above. The resulting plate was of a high quality, having a speckled brown appearance. The plate is shown in Figure 3. Example 4 A clay composition for forming ceramic articles was prepared from the following components: Porcelain Ceramic Waste 60g Porcelain Clay 760g Organic Ash 40g The ceramic waste was prepared by grinding fired porcelain clay in a ball mill to form a powder. The ground material was sieved using a 60 mesh sieve to provide a powder. The clay employed was a waste porcelain clay, that is dried, unfired porcelain clay. The clay was ground in a ball mill to form a powder. The ground material was sieved using a 60 mesh sieve to provide a powder. The organic ash employed was prepared by burning coal formed from olive stones. The organic ash was ground in a ball mill and the ground material sieved using a 60 mesh sieve to form a powder. The dry powdered ceramic waste, clay and wood ash were combined and mixed with the water. The amount of water was equivalent to an amount of 30% volume of the mixture. The resulting mixture was mixed by hand and wedged in conventional manner. The resulting material was rolled into a slab on a bat and thrown on a potter’s wheel to form a plate, again in conventional manner. The plate was removed from the bat and placed on a wooden board to fully dry. The dried plate was glazed using a ceramic spray-gun in a booth and allowed to dry. The dried, glazed plate was fired in a kiln at a temperature of 1220°C in known manner. The resulting plate was of a high quality, having a speckled white appearance. The plate is shown in Figure 4. Example 5 A clay composition for forming ceramic articles was prepared from the following components: Terracotta Ceramic Waste 85g Organic Ash 20g The ceramic waste was prepared by grinding fired terracotta clay in a ball mill to form a powder. The ground material was sieved using a 60 mesh sieve to provide a powder. The organic ash employed was prepared by burning coal formed from olive stones. The organic ash was ground in a ball mill and the ground material sieved using a 60 mesh sieve to form a powder. The components were used to prepare a tile by moulding using the general method of Example 1 above. As an alternative to moulding, the tile may be cut from a slab of the workable clay material. The resulting tile was of a high quality, having a dark speckled brown appearance. The tile is shown in Figure 5. Example 6 A clay composition for forming ceramic articles was prepared from the following components: Porcelain Ceramic Waste 85g Organic Ash 15g Water 60g The ceramic waste was prepared by grinding fired porcelain clay in a ball mill to form a powder. The ground material was sieved using a 60 mesh sieve to provide a powder. The organic ash employed was prepared by burning coal formed from olive stones. The organic ash was ground in a ball mill and the ground material sieved using a 60 mesh sieve to form a powder. The components were used to prepare a tile by moulding using the general method of Example 1 above. As an alternative to moulding, the tile may be cut from a slab of the workable clay material. The resulting tile was of a high quality, having a pale, speckled brown appearance. The tile is shown in Figure 6. Example 7 A clay composition for forming ceramic articles was prepared from the following components: Buff Stoneware Ceramic Waste 60g Glaze waste 10g Organic Ash 30g Water 60g The ceramic waste was prepared by grinding fired terracotta clay in a ball mill to form a powder. The ground material was sieved using a 60 mesh sieve to provide a powder. The glaze waste was unfired glaze remaining as waste after glazing ceramic items. The organic ash employed was prepared by burning coal formed from olive stones. The organic ash was ground in a ball mill and the ground material sieved using a 60 mesh sieve to form a powder. The components were used to prepare a brick by moulding using the general method of Example 1 above. As an alternative to moulding, the brick may be cut from a slab of the workable clay material. T1 The resulting brick was of a high quality, having a speckled beige appearance. The tile is shown in Figure 7. 5

Claims

1. A composition for forming ceramic articles, the composition comprising:a finely divided fired ceramic material; andan unfired clay;wherein the unfired clay has higher plasticity than the ceramic material; andwherein the finely divided fired ceramic material forms the major portion of the composition.

2. The composition according to claim 1, wherein the finely divided fired ceramic material has a particle size of from mesh 140 (105 microns) to mesh 40 (420 microns).

3. The composition according to either of claims 1 or 2, wherein the finely divided fired ceramic material is derived from the firing of one or more of the following clays: Grogged Stoneware, Buff Stoneware, Crank Stoneware, White Stoneware, Black Stoneware, Pink Stoneware, Red Stoneware, Speckle / flecked Stoneware, Sandstone stoneware, Grey Stoneware, Vulcan Black Stoneware, Special Stoneware, High Fire Terracotta, Oxidising St Thomas, Reduction St Thomas, Parian, Porcelain, Modelling Clay, Architectural Clay, Handbuilding Clay, Pizza Oven Clay, Lavafleck, KGM Body, and Ball Clay.

4. The composition according to any preceding claim, wherein the finely divided ceramic material is present in an amount of from 60% to 98% by weight.

5. The composition according to any preceding claim, wherein the unfired clay is a finely divided material having a particle size of from mesh 140 (105 microns) to mesh 40 (420 microns).

6. The composition according to any preceding claim, wherein the unfired clay is present in an amount of from 1.5% to 45% by weight.

7. The composition according to any preceding claim, wherein the unfired claycomprises one or more of: Grogged Stoneware, Buff Stoneware, Crank Stoneware, White Stoneware, Black Stoneware, Pink Stoneware, Red Stoneware, Speckle / flecked Stoneware, Sandstone stoneware, Grey Stoneware, Vulcan Black Stoneware, SpecialStoneware, High Fire Terracotta, Oxidising St Thomas, Reduction St Thomas, Parian, Porcelain, Modelling Clay, Architectural Clay, Handbuilding Clay, Pizza Oven Clay, Lavafleck, KGM Body, and Ball Clay.

8. The composition according to any preceding claim, wherein the unfired clay has a firing temperature that allows the composition to fired at a temperature above 1200°C.

9. The composition according to any preceding claim, wherein the unfired clay comprises a montmorillonite clay.

10. The composition according to claim 9, wherein the montmorillonite clay is present in an amount of from 3 to 8% by weight.

11. The composition according to any preceding claim, wherein the unfired clay material comprises a mixture of a first unfired clay consisting of montmorillonite and a second unfired clay consisting of one or more other unfired clays.

12. The composition according to claim 11, wherein the weight ratio of the first unfired clay and the second unfired clay is from 2:1 to 1:2.

13. The composition according to any preceding claim, further comprising a component having a high silica content.

14. The composition according to claim 13, wherein the component having a high silica content is an organic ash.

15. The composition according to any preceding claim, wherein the component having a high silica content is present in an amount of from 2 to 8% by weight.

16. The composition according to any preceding claim, comprising:88% by weight unfired clay material selected from Porcelain, White Stoneware, Buff, St Thomas or Terracotta;4.65% by weight wood ash; and6.98% by weight fired ceramic material derived by firing a clay selected from Porcelain, White Stoneware, Buff, St Thomas or Terracotta.

17. The composition according to any of claims 1 to 15 comprising:81 to 96% by weight fired ceramic material derived by firing a clay selected from Porcelain, Buff, St Thomas or Terracotta;0 to 9.5% by weight unfired clay selected from Porcelain, Buff, St Thomas or Terracotta;0 to 5.66% by weight bentonite; and0 to 3.77% by weight wood ash.

18. A composition for forming ceramic articles, the composition comprising:a finely divided fired ceramic material; andan organic ash.

19. A workable clay composition comprising:a clay composition according to any preceding claim; and water.

20. A method for forming a workable clay material for use in forming ceramic articles after firing, the method comprising:providing a dry composition according to any of claims 1 to 18;combining the dry composition with water; andmixing the dry composition and water to form the workable clay.

21. A clay article for firing, the clay article being formed from a workable clay composition according to claim 19.

22. The clay article for firing according to claim 21, wherein the article is self-glazing when fired, the surface of the article having been treated with a mixture of water and organic ash.1023. A method for preparing a ceramic article comprising firing a clay article according to either of claims 21 or 22.

24. A ceramic article prepared by firing a clay article according to either of claims 21 and 22.

Images