METHOD FOR HEATING MOLTENED GLASS AND GLASS OBJECT

Description

[0001] The present invention relates to a method for heating molten glass. The invention also relates to a glass article.Background of the invention

[0002] In the state of the art glass is melted and heated in a glass melting furnace. For this purpose often the glass is melted by hot gases from flames provided across the furnace above the glass surface. For heating the molten glass usually an electric current is passed through the bath of molten glass between electrodes immersed in the molten glass. Materials of the electrodes are typically chosen from molybdenum (Mo), tungsten (W) or noble metals such as Platinum (Pt), Rhodium (Rh) and Iridium (Ir).

[0003] However, such electrodes are subject to corrosion and removal of electrode material during use. Respective particles emanating from the electrodes can then also be found in the molten glass material. This in turn leads to contaminations of the final glass articles produced from the molten glass.

[0004] Such contaminations may manifest themselves in accumulations of small particles or striae of respective materials or a combination thereof in the glass material.

[0005] Thus, for obtaining class articles with reduced or even no such contaminations, so far glass material which can be molten with technologies such as microwave heating had to be used. EP 3 239 110 A1 discloses a device and a method for production of glass products from a glass melt while preventing bubble formation.

[0006] US 2006 / 144089 A1 discloses a method and apparatus for heating melts.

[0007] It is, thus, an object of the present invention to overcome the disadvantages described above with respect to the state of the art by providing means which allow a reduction of the contaminations in the glass material produced in glass melting furnaces. It is further the object of the present invention to provide glass articles of high quality.Description of the invention

[0008] The problem is solved by the invention in that a method for heating molten glass, the method comprising the steps: Providing two or more electrodes and bringing each of them at least in part in contact with the molten glass; Applying a voltage between a first electrode and a second electrode, with the voltage being an AC voltage; and Controlling the temperature of the molten glass, which comprises one or more of the following steps: i. Controlling the frequency of the applied voltage such that it is between 30 Hz and 15 kHz; and ii. Controlling the specific current load at the first and / or second electrode, such that it is 3.0 A / cm 2< or less at the surface of the electrode which has contact with the molten glass is proposed.

[0009] The invention is, thus, based on the surprising finding that the process of how the temperature of the molten glass is controlled has a significant influence on the amount of material which is removed from the electrodes and passed over to the molten glass and on what happens with the material in the molten glass. Especially the parameters referring to the frequency and the specific current load have been identified as helpful for achieving high quality glass material.

[0010] It has been proven by the inventors that applying the inventive approach allows to significantly reduce the amount of electrode material in the molten glass, thus, in the final glass products. Moreover, even if there remains electrode material of minor concentration in the molten glass, the electrode material appears to show less tendency to accumulate in the solid glass material when the inventive approach is applied. The same applies for oxides and / or alloys of these materials.

[0011] While not bound by any theory, the inventors assume that the positive effects originate from the specific heating process introduced by controlling the temperature. This is supported by the fact that only specific values of the frequency and / or of the specific current load lead to reduced contaminations such as accumulations, hence, to the high quality glass articles.

[0012] Relevant electrode material in this respect can particularly be molybdenum, platinum, iridium, tungsten, rhodium, alloys of one or more of these materials and / or oxides of one or more of these materials, and any combination thereof.

[0013] It is apparent that the present invention can be applied to any conventional glass melting furnace which employs heating by means of electrodes. This allows retrofitting any existing heating process by simply introducing a temperature control mechanism as proposed by the invention. This can be accomplished easily and without high costs which is very economic.

[0014] Thus, even if there are still minor amounts of electrode material present in the molten glass, it has been successfully achieved that no accumulations, hence, no striae are present any longer in the cooled glass.

[0015] The glass produced with the proposed method allows to produce glass articles of high quality. For example, for producing glass material used in the field of pharmaceutical containers, the proposed method may be employed. Especially for pharmaceutical containers such as vials, syringes, cartridges, ampoules and the like it is advantageous to have glass material of high quality. High quality ensures that pharmaceutical compositions held by the containers do substantially not get in contact with any contaminations.

[0016] In one embodiment, the specific current load is an average specific current load over the surface of the respective electrode which has contact with the molten glass.

[0017] Preferably, each electrode has a surface area but only a part of the surface area has contact with the molten glass. It is then preferred that said part of the surface area is relevant for the specific current load.

[0018] In one embodiment, the specific current load at the first and / or second electrode is obtained with respect to a surface of the respective electrode which equals a fraction of 2 / 3 of its geometric electrode surface. In other words, the specific current load at the first and / or second electrode is obtained with respect to a surface A eff which is equal to 2 / 3*A geom , wherein A geom is the geometric surface of the respective electrode. Preferably said surface A eff is used for electrodes or pair of electrodes which are of the form of rod electrodes, especially if the electrodes do not heat tip to tip, but across. For example, an electrode in form of a cylinder of height h and radius r has a geometric surface of A geom =h*2*PI*r+2*PI*r 2< .

[0019] In one embodiment the specific current load at the first and / or second electrode is obtained with respect to a surface of the respective electrode which equals the geometric surface A geom of the respective electrode, especially if the electrodes heat tip to tip and / or if the electrodes are in form of a sphere. For example, an electrode in form of a sphere of radius r has a geometric surface of A geom =4*PI*r 2< .

[0020] Preferably, in case the respective electrode is or comprises a rod electrode, the specific current load is determined with respect to 66 % (=2 / 3) of the entire contact surface of that electrode. This might be viewed as the "active" surface of the electrode in the heating scenario of a heating circuit comprising a pair of electrodes. Hence, the part of the electrode surface not facing the other one of the pair of electrodes has less weight.

[0021] Alternatively or in addition, controlling the specific current load at the first and / or second electrode may comprise that the respective specific current load is 0.01 A / cm 2< or more, 0.05 A / cm 2< or more, 0.1 A / cm 2< or more, 0.2 A / cm 2< or more, 0.3 A / cm 2< or more, 0.4 A / cm 2< or more, 0.5 A / cm 2< or more, 1.0 A / cm 2< or more, 1.5 A / cm 2< or more, 2.0 A / cm 2< or more or 2.5 A / cm 2< or more at the surface of the electrode which has contact with the molten glass.

[0022] Alternatively or in addition, controlling the specific current load at the first and / or second electrode may comprise: Controlling the specific current load at the first and / or second electrode, such that it is 2.5 A / cm 2< or less, 2.0 A / cm 2< or less, 1.5 A / cm 2< or less, 1.0 A / cm 2< or less, 0.5 A / cm 2< or less, or 0.3 A / cm 2< or less at the surface of the electrode which has contact with the molten glass.

[0023] Alternatively or in addition, controlling the specific current load at the first and / or second electrode may comprise that the specific current load at the first electrode and the specific current load at the second electrode are different. Especially there is a first specific current load at the first electrode and a second specific current load, which is different compared to the first specific current load, which especially is higher or lower, at the second electrode. For each of the specific current load, such as the first specific current load and the second specific current load, one or more limits of the specific current load as stated above may be chosen, especially an upper limit and / or a lower limit.

[0024] For example, the current load at the first electrode may be controlled such that it is 2.0 A / cm 2< or less while the current load at the second electrode may be controlled such that it is 0.5 A / cm 2< or less. This means, each specific current load here has an upper limit.

[0025] For example, the current load at the first electrode may be controlled such that it is 2.0 A / cm 2< or less and 0.5 A / cm 2< or more while the current load at the second electrode may be controlled such that it is 0.5 A / cm 2< or less and 0.1 A / cm 2< or more. This means, each specific current load here has an upper limit and a lower limit.

[0026] In preferred embodiments there are two or more pairs of first and second electrodes employed. In this situation the frequency of the voltage applied between each pair and / or the specific current load at each of the further first and / or second electrode may be controlled individually, likewise as described herein, especially as described above with respect to the first and second electrodes.

[0027] A very flexible setup is possible, if two or more pairs of electrodes, especially pairs of first and second electrodes, are used within the method. Since the electrodes of all pairs of electrodes may contribute to the contamination of the glass, it is preferred that some or even all electrodes are operated according to the invention.

[0028] In further preferred embodiments of the method, one or more third electrodes or one or more pairs of third electrodes are provided and the method further comprises: Bringing each of them at least in part in contact with the molten glass. The method then may further comprise: Applying a voltage between the first electrode, the second electrode, one, two or more of the third electrodes, or any combination of these electrodes, with the voltage being an AC voltage. The frequency and / or the specific current load with respect to these third electrodes may then be controlled likewise as it has been described herein with respect to the first and second electrodes. Especially the frequency and / or the specific current load may be controlled individually for one or more or even all of the third electrodes.

[0029] In one embodiment, different pairs of electrodes might be operated with different specific current loads, especially with different first and second specific current loads.

[0030] In one embodiment, different pairs of electrodes might be operated with different frequencies. For example there might be at least two pairs of electrodes, such as two pairs of first and second electrodes. One pair might then be operated with a first frequency, such as 50 Hz, and another pair might then be operated with a second frequency, such as 10 kHz.

[0031] In one embodiment there are 2 or more, 5 or more, 10 or more, 15 or more, 20 or more, 25 or more, 30 or more, 35 or more, 40 or more, or 50 or more pairs of electrodes. Alternatively or in addition there are 100 or less, 80 or less, 50 or less, 40 or less, 30 or less, 20 or less or 10 or less pairs of electrodes.

[0032] In one embodiment there might be in total 2 or more, 5 or more, 10 or more, 15 or more, 20 or more, 25 or more, 30 or more, 35 or more, 40 or more, 45 or more, 50 or more, 55 or more, 60 or more, 70 or more, 80 or more, 90 or more, or 100 or more electrodes, especially first, second and / or third electrodes. Alternatively or in addition there are 200 or less, 100 or less, 80 or less, 50 or less, 40 or less, 30 or less, 20 or less or 10 or less of electrodes.

[0033] If there are provided a plurality of electrodes, and especially if the electrodes are at least in part be controlled individually, a highly precise heating procedure can be provided. This allows that glass material of particular high quality can be obtained.

[0034] It might be, of course, also possible in preferred embodiments that the frequency and / or the specific current load is / are controlled for some or all of the electrodes individually, in line with the ranges provided herein for the frequency and the specific current load.

[0035] In one embodiment, one or more, especially all, of the electrodes may belong to the same heating circuit, especially in a heating apparatus for heating molten glass.

[0036] In one embodiment, all or some of the electrodes may have different effective areas.

[0037] The electrodes may be arranged in one single melting area or may be distributed across two or more different melting areas, which especially are separated from each other.

[0038] In one embodiment it is preferred that the first electrode is a rod electrode and / or the second electrode is part of a segment of a wall, which wall preferably defines at least in part a volume for holding the molten glass, wherein especially the molten glass contacts at least parts of the wall while applying a voltage between the first and second electrodes.

[0039] A rod electrode provides a high degree of freedom to be arranged in the glass melting furnace. If an electrode is designed as part of a segment of the wall, the electrodes can be provided within a compact setup.

[0040] In one embodiment it is preferred that controlling the frequency of the applied voltage comprises controlling the frequency of the applied voltage such that it is between 30 Hz and 100 Hz or between 1 kHz and 11 kHz, especially between 9 kHz and 11 kHz; and / or Controlling the specific current load at the first and / or second electrode comprises controlling the specific current load at the first and / or second electrode, such that it is 1.0 A / cm 2< or less, preferably 0.5 A / cm 2< or less, more preferably 0.3 A / cm 2< or less.

[0041] It has been proven to be beneficial that the frequency and the specific current load is chosen based on the specific situation and the respective step carried out in the process of preparing the glass material. For example if the molten glass is heated during the late process step of refining the molten glass, lower specific current loads might lead to better results. Otherwise, for process steps carried out before the refining, a higher specific current load might be preferred.

[0042] Without being bound to any theory, the inventors assume that one reason for this is that reduced specific current loads allow to "destroy" accumulation of contaminations.

[0043] Preferably controlling the frequency of the applied voltage comprises controlling the frequency of the applied voltage such that it is between 30 Hz and 100 Hz and / or controlling the specific current load at the first and / or second electrode comprises controlling the specific current load at the first and / or second electrode, such that it is 0.5 A / cm 2< or less, preferably 0.3 A / cm 2< or less.

[0044] Preferably controlling the frequency of the applied voltage comprises controlling the frequency of the applied voltage such that it is between 1 kHz Hz and 15 kHz, especially between 9 kHz and 15 kHz, more preferably between 9 kHz and 11 kHz, especially 10 kHz, and / or controlling the specific current load at the first and / or second electrode comprises controlling the specific current load at the first and / or second electrode, such that it is 2.5 A / cm 2< or less, preferably 2.0 A / cm 2< or less, 1.5 A / cm 2< or less, or 1.0 A / cm 2< or less.

[0045] Hence, it might be preferred that for a reduced frequency, such as 50 Hz, the specific current load is reduced, such as less than 0.5 A / cm 2< . This way the corrosion of the electrode can be significantly reduced. Likewise, for higher frequencies, say 10 kHz, a higher specific current load might be chosen, say less than 1.0 A / cm 2< .

[0046] In one embodiment it is preferred that the first and / or second electrodes comprise(s) molybdenum, tungsten, noble metals and / or one of the following alloys: (i) >95 weight % of Mo, (ii) >95 weight % of W, (iii) 80-100 weight % of Pt, 5-20 weight % of Rh, 0-10 weight % of Ir and 0-10 weight % of Au, and / or (iv) > 95 weight % of Ir; and / or that the first and / or second electrodes comprise(s) at least area by area an oxide layer which provides an outer surface area of the electrode, especially when the respective electrode comprises molybdenum and / or tungsten, wherein preferably the oxide layer comprises molybdenum oxide and / or tungsten oxide.

[0047] Electrodes comprising these materials have been proven well-suited for the process of melting and heating glass. If the electrode comprises an oxide layer it is more robust against the glass material, especially in case of aggressive glass materials.

[0048] In an embodiment controlling the temperature of the molten glass comprises (a) controlling that the frequency is between 9 kHz and 11 kHz and that the specific current load is 2 A / cm 2< or less or 1 A / cm 2< or less and / or (b) controlling that the frequency is between 45 Hz and 55 Hz and that the specific current load is 0.5 A / cm 2< or less or 0.2 A / cm 2< or less.

[0049] Preferably these limiting values are valid for an arbitrary arrangement and / or an arbitrary number of electrodes in a melter.

[0050] Said limiting values are preferred because this way contaminations such as material of the electrodes can significantly be reduced or even avoided.

[0051] Carrying out the method with the respective combination of values for the frequency and the specific current load leads to particularly beneficial results. Here, the contaminations are greatly reduced.

[0052] In one embodiment it is preferred that the voltage between the first and second electrodes has an amplitude of between 10 and 600 V, preferably chosen based on the glass material, especially based on the alkaline content of the glass material, and / or that the voltage is chosen such that the current flow between the first and second electrodes has an amplitude of between 50 and 2000 A, preferably of between 50 and 500 A.

[0053] Choosing the value of the voltage within the preferred range leads to particularly improved glass materials. The inventors found that the voltage value is a particularly sensitive parameter in dependence on the alkaline content of the glass material. For example for products in the pharmaceutical field, typical alkaline contents may range from 3-10 %.

[0054] Ensuring a respective current flow load leads to particularly beneficial results. Here, the contaminations are greatly reduced.

[0055] In one embodiment it is preferred that bringing each of the electrodes in contact with the molten glass comprises inserting at least one of the electrodes, preferably the first electrode, into the molten glass at least in part, and / or that bringing each of the electrodes in contact with the molten glass comprises filling the molten glass around at least a part of at least one of the electrodes, preferably the second electrode.

[0056] Inserting the electrodes allows incorporation of moveable electrodes, e.g. first filling the glass material in the respective volume for holding the molten glass and then inserting the electrodes.

[0057] Filling the molten glass around the electrodes allows incorporation of fixed electrodes, e.g. filling the glass material (partially) around them while the electrodes are arranged at least in part within the volume for holding the molten glass.

[0058] In one embodiment it is preferred that the method further comprising the step of: Controlling the duration that at least one of the electrodes has contact with the molten glass and / or separating at least one of the electrodes from contact with the molten glass after 144 hours, preferably after 48 hours, after they got in contact with the molten glass.

[0059] A precise control of the heating duration allows to achieve particular preferred glass material.

[0060] In one embodiment it is preferred that wherein controlling the duration, the voltage, the frequency and / or the specific current load leads to an amount of the material of the first and / or second electrode in the molten glass of at least 0.1 ppm, preferably of at least 0.5 ppm, but less than 50 ppm, preferably less than 30 ppm, more preferably less than 20 ppm, even more preferably less than 10 ppm, and most preferably less than 5 ppm, respectively, with respect to the weight amount of molten glass.

[0061] It is highly appreciated that the amount of electrode material can be reduced significantly by the proposed approach.

[0062] In one embodiment it is preferred that wherein the molten glass comprises 0-1 % by weight, preferably 0-0.3%, 0-500 ppm, of As 2 O 3 , 0-1 % by weight, preferably 0-0.3%, 0-1000 ppm, of Sb 2 O 3 and / or 0-1 % by weight, preferably 0-0.5%, 0-2500 ppm, of SnO 2 , respectively, as refining agent.

[0063] A respective refining agent has been proven to be advantageous for obtaining glass material of improved quality in the context of the present invention.

[0064] A value of between 2 % by weight and 3 % by weight of Fe 2 O 3 as redox buffer is preferred for production of brown glass.

[0065] A value of between 100 ppm and 800 ppm, especially of between 200 and 600 ppm, more preferably of between 400 ppm and 500 ppm, more preferably of 450 ppm, of As 2 O 3 as refining agent is preferred.

[0066] In one embodiment it is preferred that the molten glass comprises more than 0.03 % by weight and less than 1% by weight, preferably 0.03-0.5%, 0.03-0.3%, 0.03-0.2%, of one halide, such as NaCl, KCI, CaCl 2 , MgCl 2 , BaCl 2 , SrCl 2 or Z, as refining agent.

[0067] A respective refining agent has been proven to be advantageous for obtaining glass material of improved quality in the context of the present invention.

[0068] Of course, even if one type of halide is present in a preferred concentration, it is nevertheless possible that there are one or more other types of halides present as well (be it in a preferred concentration or in some other one). I.e. it is not the exclusive presence of one single halide only required.

[0069] For example typical Cl contents may be from 0.05 to 0.5 weight-% in glass, especially in case of chloride refining.

[0070] In one embodiment it is preferred that wherein the molten glass comprises between 20 ppm and 500 ppm (m / m) or between 2 % by weight and 3 % by weight, respectively, of Fe 2 O 3 as redox buffer against an oxygen reboil.

[0071] A respective redox buffer has been proven to be advantageous for obtaining glass material of improved quality in the context of the present invention.

[0072] With the proposed method it is possible to provide glass material which is of high quality and which allows to produce glass articles with said transmission factors. The glass article may be preferably directed to a pharmaceutical container. For example the glass article may be a vial, syringe, cartridge, ampoule and the like. Appropriate transmission factors, hence a respective ratio, allow to provide high quality that ensures that pharmaceutical compositions held by the containers do substantially not get in contact with any contaminations.

[0073] The relevant (electrode) materials which may cause contaminations might especially be molybdenum, platinum, iridium, tungsten, rhodium, noble metals, alloys of one or more of these materials and / or oxides of one or more of these materials.Brief description of the figures

[0074] Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of preferred embodiments, when read in light of the accompanying schematic drawings, wherein Fig. 1shows a flow diagram of a method according to the invention; Fig. 2shows a diagram indicating the amount of Mo in the solid glass material dependent on the frequency of the applied voltage and the specific current load; Fig. 3shows a glass article not according to the invention. Fig. 4shows a setup for a light transmission analysis of a glass article not according to the invention in a cross-sectional view; Fig. 5shows a perspective view of a half of the glass article as used in the light transmission analysis; and Fig. 6shows a front view of the half of the glass article along with different beam paths. Detailed description of the figures

[0075] Fig. 1 shows a flow diagram of a method 1 for heating molten glass according to the first aspect of the invention. The method 1 comprises different steps.

[0076] Step 3 is directed to providing two or more electrodes and bringing each of them at least in part in contact with the molten glass.

[0077] The first electrode is a rod electrode and the second electrode is part of a segment of a wall, which wall defines at least in part a volume for holding the molten glass, wherein the molten glass contacts the wall while applying a voltage between the first and second electrodes. The first and second electrodes comprise molybdenum. The first and second electrodes comprise at least area by area an oxide layer which provides an outer surface area of the electrode.

[0078] Bringing each of the electrodes in contact with the molten glass comprises inserting the first electrode into the molten glass at least in part and filling the molten glass around at least some part of the second electrode.

[0079] Step 5 is directed to applying a voltage between a first electrode and a second electrode, with the voltage being an AC voltage.

[0080] Step 7 is directed to controlling the temperature of the molten glass.

[0081] The step 7 comprises step 9 directed to controlling the frequency of the applied voltage such that it is between 30 Hz and 15 kHz.

[0082] The step 7 also comprises step 11 directed to controlling the specific current load at the first and second electrodes, such that it is smaller than 1.0 A / cm 2< at the surface of the electrode which has contact with the molten glass.

[0083] For example, in step 7 the frequency might be chosen such that it is between 30 Hz and 100 Hz and the specific current load might be chosen such that it is less than 0.5 A / cm 2< .

[0084] For example, in step 7 the frequency might also be chosen such that it is between 1 kHz and 15 kHz and the specific current load might be chosen such that it is less than 1.0 A / cm 2< .

[0085] Fig. 2 shows a diagram indicating the amount of Mo in the solid glass material dependent on the frequency of the applied voltage and the specific current load.

[0086] In the diagram a first curve (solid line) represents the frequency of the applied voltage. The frequency is changed at some instance of time (T3->T4) from 50 Hz to 10000 Hz and to a later instance of time (T6->T7) back to 50 Hz.

[0087] In the diagram a second curve (dashed line) represents the specific current load of less than 1.0 A / cm 2< , which is more precisely about between 0.6 and 0.8 A / cm 2< .

[0088] In the diagram a third curve (dotted line) represents the amount of Mo in the produced solid glass material.

[0089] The diagram indicates on the left vertical axis the value of the frequency for the applied voltage in Hz. The diagram indicates on the right vertical axis the value of the specific current load and the amount of Mo, in 0.1A / cm 2< and ppm, respectively.

[0090] It is apparent that the amount of Mo is strongly dependent on the frequency of the applied voltage and the specific current load at the electrodes.

[0091] For a frequency of 10 kHz apparently no particles are present, preferably for specific current loads of less than 1 A / cm 2< or independent from the specific current load.

[0092] It turned out that for frequencies of 50 Hz a specific current load of less than 0.5 A / cm 2< is preferred. And for frequencies of 10 kHz a specific current load of less than 1 A / cm 2< is preferred, however, also more than 1 A / cm 2< might be possible in further preferred embodiments.

[0093] A major difference between choosing a frequency of 50 Hz and 10 kHz may be that the creation of particles starts at a higher specific current load for 10 kHz than for 50 Hz. For example, the specific current load may be at least 60-100 % higher for 10 kHz than for 50 Hz until creation of particles starts. Therefore, choosing a higher frequency may be preferred for preventing accumulations, i.e. particles, at least for the given current load.

[0094] Fig. 3 shows a glass article 51 not according to the invention. The glass article 51, which is designed in form of a glass tube element, comprises a shell 53 which encloses a lumen 55. The shell has an outer surface 57.

[0095] For a light transmission analysis of the glass article 51, the ratio of an average amplitude transmission factor and a specific amplitude transmission factor is greater than 1.00001. Here, the contamination of the glass material which is to be analyzed might be molybdenum.

[0096] Fig. 4 shows a setup 59 for a light transmission analysis of a glass article such as the glass article 51 in a cross-sectional view

[0097] The setup 59 comprises a fixed light source 61 and a fixed detector 63. A light beam 65 is emitted from the light source 61 towards the detector 63 along a beam path 67. The setup 59 allows to determine an amplitude transmission factor of the light beam 65. The amplitude transmission factor is the factor, the amplitude of the light beam 65 is attenuated between the light source 61 and the detector 63.

[0098] The glass article 51 is divided into two halves for conducting the measurements. Fig. 5 shows one half 69 of the glass article 51 in a perspective view. The outer surface 57 of the half 69 is divided into surface areas 71 of equal shape and size. Likewise, also the other half is divided into surface areas of equal shape and size.

[0099] For every surface area 71 (of both halves of the glass article 51), the glass article 51 (or the respective half thereof) is positioned relative to the beam path 67 such that the light beam 65 propagates through the thickness of the shell 53 and crosses the respective surface area 71 perpendicularly.

[0100] It is more convenient for this measurement to have the glass article 51 divided into two halves. This allows to route the light beam 65 more easily and more accurate through the glass article 51.

[0101] Fig. 6 shows the half 69 of the glass article 51 in a front view. The surface areas 71 close to the front are indicated. Furthermore, different relative beam paths 67 are shown. Each of the beam paths 67 is perpendicular on the respective surface area 71.

[0102] Of course, Fig, 6 is only for illustration purposes. Typically, there is only one single beam path 67 and the glass article, or its half 57, is orientated appropriately so as to meet the measurement conditions with respect to perpendicularity of the light beam and the surface area for each single surface area. I.e. for each measurement the orientation of the glass article is adjusted. In the setup 59, the light source 61 and the detector 63 are on different sides of the glass article 51. But more than one beam paths are possible as well.

[0103] Furthermore, the surface areas 71 in Fig. 6 seem to have a certain depth within the shell of the glass tube element. This, however, is only for illustration purposes so as to more reliably indicate the surface elements 71 in the view of Fig. 6.

[0104] The amplitude transmission factors of all surface areas (of both halves of the glass article) are arranged within a sorted list from small values to large values. The specific amplitude transmission factor is the mean value of the first 1 % of the amplitude transmission factors arranged in the sorted list, the smallest one being among the first 1 % of the amplitude transmission factors. The average amplitude transmission factor is the mean value of all amplitude transmission factors arranged in the sorted list.List of reference numerals

[0105] 1Method 3 - 11Step 51Glass article 53Shell 55Lumen 57Surface 59Setup 61Light Source 63Detector 65Light beam 67Beam path 69Half 71Surface area

Claims

1. A method for heating molten glass, the method comprising the steps: - Providing two or more electrodes and bringing each of them at least in part in contact with the molten glass; - Applying a voltage between a first electrode and a second electrode, with the voltage being an AC voltage; and - Controlling the temperature of the molten glass, which comprises one or more of the following steps: i. Controlling the frequency of the applied voltage such that it is between 30 Hz and 15 kHz; and ii. Controlling the specific current load at the first and / or second electrode, such that it is 3.0 A / cm2 or less at the surface of the electrode which has contact with the molten glass.

2. Method according to claim 1, wherein the first electrode is a rod electrode and / or the second electrode is part of a segment of a wall, which wall preferably defines at least in part a volume for holding the molten glass, wherein especially the molten glass contacts at least parts of the wall while applying a voltage between the first and second electrodes.

3. Method according to any one of the preceding claims, wherein Controlling the frequency of the applied voltage comprises controlling the frequency of the applied voltage such that it is between 30 Hz and 100 Hz or between 1 kHz and 11 kHz, especially between 9 kHz and 11 kHz; and / or Controlling the specific current load at the first and / or second electrode comprises controlling the specific current load at the first and / or second electrode, such that it is 1.0 A / cm2 or less, preferably 0.5 A / cm2 or less, more preferably 0.3 A / cm2 or less.

4. Method according to any one of the preceding claims, wherein the first and / or second electrodes comprise(s) molybdenum, tungsten, noble metals and / or one of the following alloys: (i) >95 weight % of Mo, (ii) >95 weight % of W, (iii) 80-100 weight % of Pt, 5-20 weight % of Rh, 0-10 weight % of Ir and 0-10 weight % of Au, and / or (iv) > 95 weight % of Ir; and / or wherein the first and / or second electrodes comprise(s) at least area by area an oxide layer which provides an outer surface area of the electrode, especially when the respective electrode comprises molybdenum and / or tungsten, wherein preferably the oxide layer comprises molybdenum oxide and / or tungsten oxide.

5. Method according to any one of the preceding claims, wherein the voltage between the first and second electrodes has an amplitude of between 10 and 600 V, preferably chosen based on the glass material, especially based on the alkaline content of the glass material, and / or wherein the voltage is chosen such that the current flow between the first and second electrodes has an amplitude of between 50 and 2000 A, preferably of between 50 and 500 A.

6. Method according to any one of the preceding claims, wherein bringing each of the electrodes in contact with the molten glass comprises inserting at least one of the electrodes, preferably the first electrode, into the molten glass at least in part, and / or wherein bringing each of the electrodes in contact with the molten glass comprises filling the molten glass around at least a part of at least one of the electrodes, preferably the second electrode.

7. Method according to any one of the preceding claims, further comprising the step of: - Controlling the duration that at least one of the electrodes has contact with the molten glass and / or separating at least one of the electrodes from contact with the molten glass after 144 hours, preferably after 48 hours, after they got in contact with the molten glass.

8. Method according to any one of the preceding claims, wherein controlling the duration, the voltage, the frequency and / or the specific current load leads to an amount of the material of the first and / or second electrode in the molten glass of at least 0.1 ppm, preferably of at least 0.5 ppm, but less than 50 ppm, preferably less than 30 ppm, more preferably less than 20 ppm, even more preferably less than 10 ppm, and most preferably less than 5 ppm, respectively, with respect to the weight amount of molten glass.

9. Method according to any one of the preceding claims, wherein the molten glass comprises 0-1 % by weight, preferably 0-0.3%, 0-500 ppm, of As2O3, 0-1 % by weight, preferably 0-0.3%, 0-1000 ppm, of Sb2O3 and / or 0-1 % by weight, preferably 0-0.5%, 0-2500 ppm, of SnO2, respectively, as refining agent.

10. Method according to any one of the preceding claims, wherein the molten glass comprises more than 0.03 % by weight and less than 1% by weight, preferably 0.03-0.5%, 0.03-0.3%, 0.03-0.2%, of one halide, such as NaCl, KCI, CaCl2, MgCl2, BaCl2, SrCl2 or Z, as refining agent.

11. Method according to any one of the preceding claims, wherein the molten glass comprises between 20 ppm and 500 ppm (m / m) or between 2 % by weight and 3 % by weight, respectively, of Fe2O3 as redox buffer against an oxygen reboil.