Process for treating a decorated porcelain article and its implementation device

Abstract

A process for treating a decorated porcelain article (1) comprising a glaze layer (5) over a decoration (4), the process comprising, after manufacture of said decorated porcelain article, at least one chemical tempering step and preferably a preliminary heat treatment step at a temperature above the glass transition temperature of the glaze (5). Figure to be published with the abbreviation: Fig. 1

Description

Title of the invention: Method for treating a decorated porcelain article and its implementation device

[0001] The invention relates to a method for treating a decorated porcelain article, more particularly a decorated article intended for food contact, as well as the device for implementing such a treatment method.

[0002] The invention will be described more particularly with regard to a process for articles made of so-called hard porcelain, in particular porcelain based on at least 25% kaolin, notably at least 50% kaolin, and comprising, after manufacture, mullite, quartz, and a vitreous phase. Hard porcelain is characterized by a coefficient of thermal expansion between 20°C and 300°C of less than 60 × 10⁷ K⁻¹.

[0003] A decorated hard-paste porcelain piece consists of a multilayered material comprising a hard-paste porcelain base (a few millimeters thick), a transparent enamel layer (a few hundred microns thick), one or more ceramic color layers (a few tens of microns thick) constituting the decoration, and a top layer, known as the glaze, which is colorless and transparent (a few microns thick). The manufacturing process involves stacking the different layers of vitreous materials, which also have varying geometric and thermomechanical properties (thickness, coefficient of expansion, glass transition temperature, etc.), and bonding them together through successive heat treatments.

[0004] For decoration, various techniques exist, the most common being the application of ceramic decals (chromos) obtained by chromolithography. This process allows the use of a very wide range of colors, combined with the possibility of fine and complex designs. The decoration takes the form of a decal obtained by printing one or more layers of ceramic inks and a polymer matrix. After cutting out the decal, this decoration is precisely applied to the piece to be decorated and then undergoes appropriate heat treatment. This heat treatment eliminates the organic fraction and consolidates the ceramic inks by partially fusing them with the usual transparent glaze covering the raw porcelain to create colored glazes.

[0005] However, these colored enamels obtained after firing the inks consist of a mixture of silica, boron, alumina, alkali and alkaline earth oxides, and trace metal oxides (TMEs), which are dissolved in the glass matrix (titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, cadmium, rare earths, etc.) or trapped in mineral pigments (CdS, CdSe, CoA12O4, ZrSiO4, etc.). These metallic elements tend to be released during use. Decorated items can, over time, be harmful to the health of users when the items come into contact with food. This is why a coating is applied over the colored decoration; this coating acts as a barrier layer to limit the release of such metallic elements into food. However, the integrity of the coating (and the decorative layers) is often compromised.

[0006] Furthermore, European standards will be strengthened with regard to the risk of trace metals released by ceramic materials that come into contact with food. As early as 2004, European Regulation EC No. 1935 / 2004 specified that "materials and articles placed or intended to be placed in contact with food must be inert with respect to food." And since 2012, the European Commission has been preparing legislative changes aimed in particular at drastically lowering the release thresholds for lead and cadmium in decorated porcelain objects and also targeting other chemical elements such as cobalt, chromium, barium, aluminum, nickel, arsenic, etc.

[0007] Thus, today, decorated porcelain articles intended for food contact may no longer meet future European standards.

[0008] To overcome this drawback of the release of trace elements, French patent FR3104570 proposes a solution by integrating, during the manufacturing process of the porcelain article, the incorporation of an intermediate layer between the glazed porcelain support and the decoration layer, whose coefficient of thermal expansion is between the two coefficients of thermal expansion of the underlying transparent enamel and overlying decoration layers, which provides a thermal expansion gradient damper making it possible to reduce the thermomechanical stresses between the porcelain support and the decoration and to reduce the appearance of cracks in the decoration down to the glaze.

[0009] However, this solution requires modifying the traditional manufacturing process of decorated porcelain, by adding intermediate deposition and firing steps and by requiring the procurement of a specific material with respect to the desired coefficient of thermal expansion.

[0010] The present invention therefore aims to propose another solution to drastically reduce the release of ETM from decorated porcelain, in particular from a decorated hard porcelain article, which also allows not only the traditional manufacturing process of decorated porcelain to be preserved but also to be simple to implement.

[0011] According to the invention, the process for treating a decorated porcelain article comprising a glaze layer over a decoration includes, after manufacture of said decorated porcelain article, at least one chemical tempering step. More particularly, the porcelain article comprises a base, a glaze covering the The process involves applying at least one surface of the substrate, a design applied to the enamel, and a glaze covering the design. The treatment method of the invention improves the barrier function of the glaze. Chemical hardening is achieved by coating at least one surface of the glaze with a chemical hardening composition (in liquid or paste form, similar to a slip). The process could include several chemical hardening steps on the same article.

[0012] Surprisingly, chemical tempering applied to decorated porcelain reduces the release of ETM without damaging or degrading the glaze or decoration.

[0013] Unexpectedly, the inventor highlighted a new use of chemical tempering, namely to reduce the release of ETM from the glaze of a decorated porcelain, and this as soon as the process of the invention was implemented, regardless of the use of the porcelain article over time and its resistance to cracking over time.

[0014] While chemical hardening is known to strengthen glazed (but undecorated) ceramics such as "Bone China," which has a coefficient of thermal expansion between 20°C and 300°C greater than 60 x 10⁷ K⁻¹, it was not obvious to immediately perform chemical hardening on porcelain, which has a much lower coefficient of thermal expansion, and moreover, on decorated porcelain, at the risk of damaging the decoration. Indeed, the glaze of a decorated porcelain item is only a few microns thick compared to the transparent glaze of several hundred microns that covers a ceramic, especially an undecorated one. However, contrary to expectations, the resulting glaze has a much more effective barrier layer function, with tests showing a reduction in the release of trace metals by a factor of at least two, or even more.

[0015] Furthermore, the process of the invention has the advantage of being implemented after the usual manufacturing process for decorated porcelain (therefore, there is no need to modify the traditional manufacturing steps). Moreover, chemical tempering is a simple operation, well understood by those skilled in the art. Finally, the process of the invention has the further advantage of being able to be implemented both immediately after manufacturing and at a later time, even after use. Thus, the treatment process of the invention can be applied to decorated porcelain that has been stored or already used and that would need to be treated to meet new standards.

[0016] Preferably, the process includes, prior to chemical hardening, a heat treatment step (at least of the coating, preferably of the entire article) at a temperature that is higher than the glass transition temperature of the coating. The inventor has unexpectedly demonstrated that chemical hardening combined with a pre-heat treatment at a suitable temperature that If the temperature is found to be above, rather than below, the glass transition temperature of the coating, drastically reduces the release of trace elements (TEs). Tests have shown a reduction in TE release of up to a factor of 45.

[0017] According to one feature, chemical hardening is achieved by applying, at least to the coating of the article, a liquid or paste-like chemical hardening composition based on salts capable of inducing ion exchange at the coating level. This chemical hardening is carried out at a treatment temperature between the melting point of the salts in the chemical hardening composition and the glass transition temperature of the coating, in particular at a treatment temperature between 350 and 550°C. The glass transition temperature is considered here to be the temperature separating the material's solid behavior from its viscous liquid behavior. This temperature is revealed during a dilatometry measurement on a bulk sample of the glassy material, at a heating rate of 5°C / min, by a sudden increase in the material's thermal expansion.This glass transition temperature is defined here as the intersection of the tangents of the solid expansion and the liquid expansion at this discontinuity.

[0018] Advantageously, the chemical quenching composition comprises alkali salts such as nitrates and / or sulfates and / or chlorides and / or phosphates and / or borates and / or carbonates, selected from sodium and / or potassium and / or rubidium and / or cesium.

[0019] According to one embodiment, chemical hardening is carried out by immersing the article in a bath of a chemical hardening liquid composition. The composition is directly at the treatment temperature or is at least at the melting temperature of the salts and is raised to the treatment temperature.

[0020] According to another embodiment, chemical hardening is carried out by depositing on at least the coating of the article, a chemical hardening paste composition in the form of a slip, in particular the slip being suitable for spraying or applying with a brush, and then by heating, at least of the coating, preferably of the whole article, to a chemical hardening temperature; this is a heating to a temperature corresponding to that of the chemical hardening treatment or a heating by contact with an atmosphere which is directly at the temperature of the chemical hardening treatment.

[0021] Preferably, the process includes, prior to chemical tempering, a heat treatment step, at least of the coating, preferably of the entire article. Advantageously, the heat treatment temperature is between 550°C and 850°C, preferably between 600°C and 750°C. Care must be taken to ensure that the treatment temperature does not exceed the firing temperature of the decoration. If The operator knows this firing temperature and will immediately define the maximum processing temperature. If this is not the case, particularly when the item was manufactured in another factory and / or is old, the operator, knowing from their general technical knowledge that a decoration is fired between 800°C and 950°C, will impose a heat treatment temperature that does not exceed 750°C.

[0022] When the process includes, prior to chemical hardening, advantageously a heat treatment step, the process necessarily includes, between the heat treatment and the chemical hardening, a cooling step of the article, in particular between 1 and 20°C / minute, in particular, when the chemical hardening is carried out by immersion of the article in a chemical hardening bath, the article is cooled to a temperature corresponding to that of the bath or to a temperature not exceeding 50°C above the temperature of the bath, and when the chemical hardening is carried out by application of a chemical hardening slurry, the article is cooled to a temperature not exceeding 150°C, in particular is cooled to a temperature between 20°C and 150°C, preferably to a temperature between 50°C and 95°C.

[0023] According to another feature, the process includes, after the chemical tempering step, a cooling step (of the cover or of the whole article), in particular between 1 and 20°C / minute, then a washing step.

[0024] Advantageously, the process includes, before the chemical tempering step or before the heat treatment step in the case of heat treatment prior to chemical tempering, a washing step and a drying step, preferably these preliminary washing and drying steps also being carried out when the treatment process is implemented immediately after the manufacturing step of the decorated porcelain.

[0025] According to another feature, the decorated porcelain article comprises a support made of hard porcelain. In particular, the decorated porcelain article on which the process is carried out comprises a porcelain support that is based on at least 25% kaolin, in particular at least 50% kaolin, and comprises, after manufacture, mullite, quartz and a vitreous phase.

[0026] According to yet another characteristic, the decorated porcelain article comprises a support made of porcelain and based on at least 25% kaolin, being free from calcined bone powder or calcium phosphate. According to this characteristic, the treatment process is thus applied to hard-paste porcelain and not to bone china.

[0027] According to another feature, the decorated porcelain article on which the process is carried out comprises a porcelain substrate whose coefficient of thermal expansion between 20°C and 300°C is less than 60 × 10⁷ K*. The coefficient of thermal expansion is estimated by the dilatometry method at a speed of 5°C / min.

[0028] The invention further relates to a decorated porcelain article comprising, on the one hand, at least one support, in particular a support based on at least 25% kaolin, preferably based on at least 50% kaolin, and comprising mullite, quartz and a vitreous phase, and on the other hand a layer of transparent enamel, a decoration and a glaze, the article having undergone the aforementioned treatment process of the invention.

[0029] The invention also relates to the use of chemical tempering on decorated porcelain to limit the release of metallic trace elements (MTEs).

[0030] Finally, the invention relates to a device for implementing the aforementioned treatment process of the invention on one or more decorated porcelain articles, the device comprising at least one heating chamber such as a kiln for carrying out the chemical tempering step, the chamber including a chemical tempering bath when the chemical tempering is carried out by immersion in a chemical tempering bath. In a preferred example of the device, the device comprises a heat pretreatment chamber and a chemical tempering chamber, the chambers being equipped with heating elements and preferably with temperature sensors, the device further comprising a chamber separation element, such as a movable and / or removable hatch or partition, and preferably automated transport means capable of conveying a support for the articles to be treated, such as a basket, from one chamber to the other.Advantageously, the device includes a system for controlling and regulating the temperatures of said heating elements within the enclosure.

[0031] Some features and other advantages of the invention will become apparent from the following description, given solely by way of non-limiting example, and made with reference to the accompanying drawings, in which: - [Fig.l] represents a partial cross-sectional view of a decorated porcelain item. - [Fig.2] is a schematic cross-sectional view of an example of an implementation of device for implementing the treatment process according to the invention including the possibility of thermal pretreatment, the articles being arranged here in a thermal pretreatment chamber. - [Fig.3] corresponds to [Fig.2] with regard to the chemical quenching step.

[0032] The process of treating a decorated porcelain article of the invention applies more specifically to an article intended to be in contact with foodstuffs, such as plates, bowls, cups, dishes, salad bowls, etc.

[0033] The processing method of the invention is a traditional post-fabrication process for an everyday decorated porcelain article such as article 1 illustrated in [Fig. 1]. The decorated porcelain article 1 is made of hard porcelain. The decorated porcelain article 1 comprises a multilayer material which includes a porcelain support 2 hard, a layer of transparent enamel 3, a decoration 4 which can be multi-layered, and a glaze 5.

[0034] The treatment process of the invention can also be applied to an article comprising additional layers between the enamel layer 3 and the decoration 4.

[0035] The porcelain support 2 considered in the embodiments described below is made of hard porcelain. The support 2 is manufactured from a ceramic material comprising at least 25% kaolin, preferably at least 50% kaolin, as well as a mixture of quartz, feldspar, and other additives. After firing at a temperature above 1320°C, the hard porcelain support 2 comprises a vitreous phase, mullite, and quartz. In particular, the support 2 has a coefficient of thermal expansion between 20°C and 300°C of less than 60 × 10⁷ K⁻¹.

[0036] The support 2 has a thickness between 0.1 and 200 mm, in particular for tableware, a thickness between 1 and 10 mm.

[0037] The transparent enamel layer 3, which can be colorless or colored, is a glass layer which has a glass transition temperature above 600°C and a coefficient of expansion between 20°C and 300°C lower than that of the support 2.

[0038] The transparent enamel layer 3 has a thickness of 0.05 mm to 5 mm, preferably a thickness between 0.1 mm and 1 mm.

[0039] Decor 4 comprises one or more layers of colorless or colored glass, more or less loaded with ceramic particles.

[0040] The decoration 4 has a glass transition temperature that is above 400°C and below that of the enamel layer 3. The decoration 4 has a coefficient of expansion between 20°C and 300°C greater than that of the substrate 2. In the case of a multilayer decoration, each of the layers has a coefficient of expansion between 20°C and 300°C greater than that of the substrate 2.

[0041] Decor 4 has a total thickness (of the single layer or of all the layers of the decor) of between 1 pm and 500 pm, in particular between 5 pm and 50 pm.

[0042] Coating 5 is a colorless and transparent layer of glass. Coating 5 comprises, for example, but not limited to, elements such as LiO2 and / or Na2O, Al2O3, B2O3 and SiO2.

[0043] The cover 5 has a glass transition temperature which is greater than 400°C.

[0044] The cover 5 has a thickness between 1 pm and 500 pm, in particular between 5 pm and 50 pm.

[0045] The treatment process of the invention is to act after manufacture of article 1, either immediately after its manufacture and before use, or subsequently and after use, by one or more treatment operations on the cover 5 with barrier layer function.

[0046] The treatment process comprises the following steps: - advantageously a wash; - preferably a pre-heat treatment at a temperature higher than the glass transition temperature of the coating; - a chemical tempering treatment; and - preferably a washing.

[0047] Since the process can be carried out well after the manufacture of article 1, it is preferable to wash article 1, at least cover 5. Preferably, the washing step is carried out immediately after the article 1 is manufactured and ready for use. The washing is done manually or by machine. Washing consists of washing the article with a cleaning solution such as water containing a degreasing agent, rinsing it, and drying it. Drying is carried out at a temperature below 150°C.

[0048] The chemical hardening treatment step consists of coating at least the coating 5 with a liquid or paste-like chemical hardening composition. The liquid chemical hardening composition is in the form of a bath (when heated to a temperature above the melting point of the salts). The paste-like chemical hardening composition is in the form of a slurry that is applied by spraying or other means of applying a paste-like composition, such as by brush. The chemical hardening composition is based on alkali salts, for example, potassium and / or sodium salts, to establish ion exchange at the coating 5. The treatment temperature is a temperature between the melting point of the salts and the glass transition temperature of the coating 5, in particular between 350°C and 550°C.The duration of the treatment is adapted to article 1, in particular according to the thickness and composition of the coating 5 and the decoration 4. The duration of the treatment is from a few minutes to several hours, or even tens of hours.

[0049] In one embodiment, chemical quenching is therefore carried out by immersion in a bath of molten alkali salts, for example potassium-based. The bath is at the aforementioned treatment temperature, in particular between 350°C and 550°C.

[0050] In another embodiment of chemical hardening, the chemical hardening composition in the form of a slip is applied to the coating by spraying or other means of application such as brushing. The chemical hardening slip is then obtained by mixing alkali salts, for example potassium-based, in water and a powdered inert agent to obtain a paste-like consistency, such as ground kaolin or ground alpha alumina. The slip is a paste that can be sprayed or applied with a brush, even on vertical surfaces. The thickness of the slip is between 100 and 5000 µm. The slip can be applied at room temperature or after preheating article 1 to a temperature not exceeding not 150°C, preferably between 50 and 95°C. Once the slip has been deposited on the glaze 5, the article is dried and then heated to the aforementioned treatment temperature, in particular between 350°C and 550°C.

[0051] After the chemical quenching step, during which article 1 was heated in contact with the chemical quenching solution, article 1 is cooled. When the article has been immersed in the chemical quenching bath and heated, it is removed, drained, and then cooled. When the article has been heated after the chemical quenching slurry has been applied, the heating is stopped. The cooling is carried out in a known manner, over a few minutes or several hours. For example, the cooling may be achieved by lowering the temperature by between 1°C and 20°C per minute.

[0052] In a preferred embodiment, a heat treatment is carried out before the chemical tempering treatment. The heat treatment is performed at a temperature above the glass transition temperature of the glaze 5. This temperature is in particular between 500°C and 850°C. The heat treatment lasts from several minutes to several hours. It is adapted in particular according to the type of glaze and its thickness, as well as the type of decoration (in relation to the firing temperature of the decoration).

[0053] When heat treatment is planned prior to chemical hardening, it will be necessary to cool article 1 beforehand. When chemical hardening is carried out by immersion, article 1 is cooled to the temperature of the chemical hardening bath or to approximately the temperature of the bath (to a temperature less than or at most 50°C above the bath temperature). When the chemical hardening composition is applied to the coating 5 in the form of a slurry rather than by immersion, the inventor has demonstrated that for good results, it is necessary to cool the article before applying the hardening slurry by spraying or brushing, to a temperature below 150°C, preferably to a temperature between 20°C and 95°C, in particular between 50°C and 95°C.

[0054] Finally, after the chemical quenching step following cooling, a washing step is performed on article 1. The washing is either manual or machine washing. It consists of washing the article with a cleaning solution such as water containing a cleaning agent, possibly rinsing it, and then drying it. The cleaning is carried out for a few minutes at a temperature below 90°C.

[0055] The treatment process of the invention can be implemented by a device 6, an example of which is schematically illustrated in [Fig. 2]. The illustrated device 6 is intended for chemical hardening by immersion in a liquid bath. The device 6 advantageously comprises a furnace chamber 60 and, combined within this chamber, a heat pretreatment chamber 61 and a chemical hardening chamber 62. The furnace chamber 60 comprises heating elements 63 arranged in each of the chambers and temperature sensors 64. The heating elements 63 and the temperature sensors 64 are connected to a temperature control and regulation system 7 which is external to the enclosure. The chemical quenching chamber 62 includes a chemical quenching bath 65. The device 6 includes a basket 66 in which several articles 1 to be treated are intended to be placed. The device 6 is designed to convey the articles 1 from the heat pretreatment chamber 61 to the chemical quenching chamber 62 without bringing the articles into contact with the ambient environment outside the chambers. For this purpose, the device 6 includes a separation element 67 between the chambers, such as a hatch or a partition, which is movable and / or removable, and preferably automated means 68 for transporting the basket 66 from one chamber to another in order to bring the basket from the heat pretreatment chamber 61 to the chemical quenching bath 65 ([Fig.3]) and to extract it after chemical quenching treatment. In the illustrated example, chemical quenching is carried out in a bath 65 of a chemical quenching solution.

[0056] Tests were carried out to demonstrate the interest of the treatment process of the invention, the results of which are summarized in Tables 1 and 2.

[0057] For each of the examples, chemical hardening is carried out either by immersion or by spraying, and may or may not undergo a pre-heat treatment: - Example 1: Article 1 comprises a support 2 in hard white glazed porcelain (glaze layer 3), a decoration 4 referenced DI consisting of a 10 µm thick colour base, and a glaze 5 referenced Cl and 9 µm thick. The article underwent chemical quenching by immersion in a liquid potassium nitrate bath KN03 at 420°C for 240 min without pre-heat treatment; - Example 2: Article 1 comprises a base 2 in hard-glazed white porcelain (glaze layer 3), a decoration 4 referenced DI consisting of a 10 µm thick color base, and a glaze 5 referenced Cl and 9 µm thick. The article underwent chemical quenching by immersion in a liquid potassium nitrate bath KN03 at 470°C for 360 min with a pre-heat treatment at 800°C (above the glass transition temperature of the glaze Cl) for 15 min. Example 2 therefore differs from example 1 by the pre-heat treatment and by the slightly higher bath temperature and the longer duration of the chemical quenching treatment; - Example 3: Article 1 comprises a base 2 of hard-glazed white porcelain (glaze layer 3), a decoration 4 referenced DI consisting of a 10 µm thick color base, and a glaze 5 referenced Cl and 9 µm thick. The article underwent chemical hardening by spraying with a chemical hardening slip containing potassium nitrate KN03 and was heated with slip at 450°C for 240 minutes with a pre-heat treatment at 800°C for 15 minutes; - Example 4: Article 1 comprises a base 2 of hard-glazed white porcelain (glaze layer 3), a decoration 4 referenced DI consisting of a 10 µm thick solid color, and a glaze 5 referenced C2, different from the glaze Cl, and 9 µm thick. The article underwent chemical hardening by immersion in a liquid potassium nitrate (KNO3) bath at 450°C for 240 minutes, with a pre-heat treatment at 600°C for 45 minutes. - Example 5: Article 1 includes a support 2 in hard white glazed porcelain (glaze layer 3), a decoration 4 referenced D2 different from the decoration DI by its thickness of 25 pm, and a glaze 5 referenced Cl. The article has undergone chemical hardening by immersion in a bath of liquid potassium nitrate KNO3 at 470°C for 240 min with a pre-heat treatment at 800°C for 15 min.

[0058] For each of Examples 1 to 5, colorimetry was measured before and after the implementation of the treatment process of the invention in order to determine whether or not the process tends to alter the colors of the decoration. The coordinates L a* b* were measured using a MINOLTA CR-300 colorimeter under Illuminant D65, at five measurement points on each piece. The color difference AE (according to the method for measuring visual difference established in 1976 by the CIE) was then calculated between the article with and without the implementation of the treatment process of the invention. The results, expressed in terms of AE, are presented in the tables. It is generally accepted that a color difference AE of less than 1 is not distinguishable by the human eye and therefore that the associated color difference is negligible, and that a color difference AE between 1 and 2 is only perceptible to an experienced eye under specific lighting.

[0059] The cobalt release rates in micrograms per kilogram of food simulant (pg / kg) were also measured for each of the example articles and for the same article corresponding to each example but not having undergone the treatment process of the invention. The test (specific migration) and measurement method consists of carrying out three successive 24-hour contact periods at 22°C of the cover 5 with a food simulant, which is a 4% concentrated acetic acid, and measuring the trace elements (TEMs) of the simulant after the third contact. The measurement of the trace elements is performed using an inductively coupled plasma mass spectrometry (ICP-MS) device. The migration gain factor of the trace elements is then calculated. For the five examples, the cobalt released rate was measured. The gain factor Co(0) / Co is the ratio between the cobalt level measured for the untreated article (Co(0)) and the cobalt level measured for the treated article (Co).The results of the gain factor Co(0) / Co. are given in the tables. For example 5, whose decoration is much thicker, the release of aluminium was also measured (by the same measurement and calculation method as for cobalt).

[0060] [Tables 1] AE Gain Factor Co(0) / Co Example 1 (Dl, Cl, immersion, without heat pretreatment) 1.9 1.7 Example 2 (Dl, Cl, immersion, with heat pretreatment) 1 7.5 Example 3 (Dl, Cl, spraying, with heat pretreatment) 0.5 10.4 Example 4 (Dl, C2, immersion, with heat pretreatment) 2.6 35.1 Example 5 (D2, Cl, immersion, with heat pretreatment) 0.5 8.4

[0061] [Tables2] Example 5 (D2, Cl, immersion, with heat pretreatment) Gain factor Co(0) / Co 8.4 Gain factor Al(0) / Al 13

[0062] The results of the treatment process of the invention are therefore very positive. In terms of color, the measurements carried out on all the examples confirm the general visual impression, namely very good color stability after the tempering test (an AE deviation of less than 3, mostly less than 2). The results of the examples show that the treatment process of the invention reduces the release of trace elements: - by applying even just a chemical quenching on example 1, we manage to reduce the release of cobalt by almost 50% (gain of 1.7) compared to an article which is not treated; - thermal pre-treatment further improves the performance of the cover as a barrier layer, as shown in example 2, whose gain factor is 7.5 compared to example 1, whose gain factor is 1.7, i.e., a gain factor multiplied by more than 4; - chemical spray hardening (with heat pretreatment) also reduces cobalt release, with the gain factor in example 3 reaching 10.4; - the nature of the coating, combined with the treatment process of the invention, can have an influence on the reduction of the release; indeed, as shown in examples 2 and 4 which undergo the same treatments (thermal pre-treatment and immersion) but with a coating of a different nature, the reduction of cobalt is multiplied by more than 4, going from 7.5 to 34.1; - moreover, we observe that with the same coating, for a thicker decoration (example 5) the result of the reduction in cobalt release remains significant at 8.4 compared to 7.5; - Finally, for example 5 with a thick layer of decoration, the treatment process of the invention also allows a very significant reduction not only of cobalt but also of aluminum (table 2)

[0063] Consequently, the treatment process of the invention leads to a decorated porcelain article whose glaze exhibits barrier layer properties that are far more effective than without chemical tempering treatment.

Claims

Demands

1. Process for treating a food contact article made of decorated porcelain (1) which comprises a support (3), an enamel (4) covering at least one face of the support, a decoration (4) deposited on the enamel and a layer of glaze (5) over the decoration (4), characterized in that the process includes after manufacture of said article made of decorated porcelain, at least one chemical tempering step.

2. A method according to claim 1, characterized in that the chemical hardening is carried out by applying, at least on the coating (5) of the article, a liquid or paste chemical hardening composition based on salts capable of leading to an ion exchange at the level of the coating, this chemical hardening being carried out at a treatment temperature between the melting temperature of the salts of the chemical hardening composition and the glass transition temperature of the coating (5), in particular at a treatment temperature between 350 and 550°C.

3. A process according to the preceding claim, characterized in that the chemical quenching composition comprises alkali salts such as nitrates and / or sulfates and / or chlorides and / or phosphates and / or borates and / or carbonates, selected from sodium and / or potassium and / or rubidium and / or cesium.

4. A method according to any one of the preceding claims, characterized in that the chemical hardening is done by immersing the article (1) in a bath of a chemical hardening liquid composition.

5. A method according to any one of claims 1 to 3, characterized in that the chemical hardening is carried out by depositing on at least the coating (5) of the article, a chemical hardening paste composition in the form of a slip, in particular the slip being suitable for spraying or brush application, and then by heating, at least of the coating, preferably of the whole of the article, to a chemical hardening temperature.

6. A method according to any one of the preceding claims, characterized in that it comprises, prior to chemical hardening, at least one heat treatment step of the coating (5), preferably of the whole article (1).

7. A method according to the preceding claim, characterized in that the heat treatment temperature is between 550°C and 850°C, of Preferably between 600°C and 750°C.

8. A method according to claim 6 or 7, characterized in that it comprises, between heat treatment and chemical hardening, a cooling step of the article, in particular, when chemical hardening is carried out by immersion of the article in a chemical hardening bath, the article is cooled to a temperature corresponding to that of the bath or to a temperature not exceeding 50°C above the temperature of the bath, and when chemical hardening is carried out by application of a chemical hardening slurry, the article is cooled to a temperature not exceeding 150°C, in particular is cooled to a temperature between 20°C and 150°C, preferably to a temperature between 50°C and 95°C.

9. A process according to any one of the preceding claims, characterized in that it comprises, after the chemical quenching step, a cooling step, in particular between 1 and 20°C / minute, and then a washing step.

10. A process according to any one of the preceding claims, characterized in that it comprises, before the chemical tempering step or before the heat treatment step in the case of heat treatment prior to chemical tempering, a washing step and a drying step, preferably these preliminary washing and drying steps also being carried out when the treatment process is implemented immediately after the decorated porcelain manufacturing step.

11. A process according to any one of the preceding claims, characterized in that it is carried out on a decorated porcelain article which comprises a porcelain support which is based on at least 25% kaolin, in particular at least 50% kaolin, and comprises after manufacture, mullite, quartz and a vitreous phase.

12. A method according to any one of the preceding claims, characterized in that it is carried out on a decorated porcelain article which includes a porcelain support whose coefficient of expansion between 20°C and 300°C is less than 60 107 K1.

13. A decorated porcelain article (1) comprising, on the one hand, at least one support (2), in particular a support based on at least 25% kaolin, preferably based on at least 50% kaolin, and comprising mullite, quartz and a vitreous phase, and on the other hand, a transparent enamel layer (3), a decoration (4) and a glaze (5), the article having underwent the treatment process according to any one of the preceding claims.

14. Use of a treatment process according to any one of claims 1 to 12 on decorated porcelain to limit the release of metallic trace elements.

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