US2547149A - Method of firing porcelain - Google Patents

Description

April 3, 1951 BEAUDET METHOD OF' FIRING PORCELAIN Filed April 15, 1947 Patented Apr. 3, 1951 UNITED STATES ATEN i'OFFlCE A METHOD .OF FIRING PORCELAIN Louis Beaudet, Limoges, France, assignor to Society Bcrnardaud & Cie, Limoges, France Application April 15, 1947, Serial No. 741,651 In France April 17, 1946 locat s. (01.25-45 7 My invention has for its object a method for such an operation; that carries on for more than ten hours, can be imagined. The loading and deslagging bring insubstantial modifications in the composition of the fumes, and consequently of the atmosphere surrounding the various products. This atmosphere can change from reduc ing to carburising. if the amount of air coming in The pieces first of all are biscuit baked or semi-vitrified and then enamelled, and after this they are subjected to the final burning that is carried out by a gradually increasing rise in temperature. This burning is executed in three phases.

The first phase takes place in an oxidising atmosphere up to the coating temperature that varies from 800 to 1100" 0.; this oxidising atmosphere was regarded as quite necessary to enisnot enough to ensure the conversion of the carbon in the fuel to carbon monoxide. It may change to oxidising if there is a lack of fuel at some part of the grate or an uneven distribution on it. These irregularities bring in numerous defects such as yellow or smoked impression, granulations, blisters or bubbles, dark edges, etc. all shortcomings that involve .a discharge to the waste-dumpof quite a substantial part of the production."

sure the burning of the carbon-contents in the biscuit paste or in the enamel and for avoiding any carbon deposits in the paste and enamel as a result of the dissociation of the gases resulting from the burning and distillation of the fuel.

The second phase takes place in a reducing atmosphere and is intended to ensure the reduction of the salts or the colouring metallic oxides occurring in the paste, especially iron compounds. This atmosphere is either kept stable until the end of firing, or only until the reaching of a temperature lower than that denoting the finish of the burning. This temperature at which the action of the reducing atmospherestops is often in relation with the requirements of the baking of the raw paste as semi-vitrified when this-is done by recovery of the fumes of anintermittent furnace.

If required the baking ends in an atmosphere which is neutral or slightly oxidising, often beon account ofthe recovery executed as mentioned above.

The burning is executed either in an intermittent or in a continuously operating furnace.

. causeof the'manuiacturers personal reasons or Intermittent furnaces are made as a .rule with two chambers of which one receives the semivltrified and enamelled pieces that are subjected therein to the final burning. The other chamber warmed by the fumes discharged from the first chamber, takes the pieces of raw non-enamelled paste that are baked therein to form semivitrified or biscuit pieces.

The transition from the oxidising to the reducing atmosphereis effected in ,a hearth with v solid fueLby damping the fire so as to cause the a t t rk a a a -ge r t r: 10?. em rator, a secondary air-inlet allows-the partial burning of the gases coming from'the distillation and gasification of the fuel. The trouble with With intermittent furnaces using liquid fuel the same drawbacks occur in spite of their apparent straightforwardness.'

In the straight-flow furnaces, very often of tunnel-type the methods practised vary according to the manner of heating.

When the heating is done with a volatile gas, the burners, located in the high temperature area, are regulated so as to form reducing fumes with a certain content of unburnt or partly burnt gas, expressed as an amount of carbon monoxide and hydrogen detected by chemical analysis. However, the very last burners, in the direction of the flow of the products, may be regulated as neutral or oxidising in order to produce the third phase mentionedabove.

In the drawing,

Fig. 1 is a diagrammatic view illustrating the novel method of the present invention; and

.Fig. 2 is a temperature curve obtained from practicing the method of the invention.

Referring to the diagram of Fig. 1,,the fumes traversingthe area CB are reducing. At B, the point at which thetemperature is kept close to coating point, hotair is injected intothe fumes in .Orderto burn off .the excessof volatile gas and to makej'the atmosphere oxidising in the area BA. In certain processes, not only is an injection made at B of hot air, but also the reducing fumes a e drawn off from the same point so that the area BA may besubjected only to an air flow. The hot air is as arule taken off that formed by the cooling of the products.

The consequence of. this is a vast complication in the furnace construction and great trouble with its regulation and its stabilization.

Apart from the temperature and the condition of the atmosphere another factor comes into the picture: the duration of the burning and of the cooling The length of time for the rise in tern-' ven ure is. co mon y. 5' h r and r. he C001. ing 3 0' hours Therejissometimes a lesser duration' required in the tunnel furnaces. I I In short, with the prevailing arrangements, it

is a matter of difficulty to regulate suitably the ranges of atmospheres that must be adjusted in the baking furnace.

The process constitutin the object of the invention aims at a cure of the drawbacks of the previous processes.

It consists in that the two main phases of the final burning that has been the practice up to the present as stated above, the first in an oxidising and the second in a reducing atmosphere, are replaced by a single phase in a reducing atmosphere.

The burning can be finished off as in the known processes by a short neutral or slightly oxidising period.

The new manner of carrying out the work is founded on the following points:

The carbon contents in the biscuit baked paste I or in the enamel, as well as that deposited during the different storings and handlings in the form of dust, grease from workmens hands etc. need not be burnt, in other words there is no necessity to convert it into carbonic gas but only to gasify same and this can be done at any temperature provided that at this temperature the paste in the process of baking is porous enough and the enamel not caked by the sintering that takes place before the meltin and vitrification. The conversion into gas of the carbon may be got by any of the following well-known reactions:

These reactions are characteristic of chemical balances and when C, C0, C02, H and H2 are present, a complex balance is set up that depends on pressure, temperature, the concentration of the elements present and-on the length of contact.

Theoretically speaking, for a sufliciently long contact period, above 700 C. the balance has a tendency to separate out carbon as given out by the equation 2CO=CO2:-C.

But the time required for reaching the balance is far above the time allowed for contact in varying atmospheres.

a tunnel furnace, so that there is little need to fear a carbon deposit. From 750 C. onwards the balance leads to the entire disappearance of carbon, as such by the reaction of carbon in carbonic acid as by the reaction on steam.

The further the temperature rises and the higher the speed of reaction increases, so that at 1000 C. the disappearance of the carbon is practically instantaneous and it becomes impossible to have any free carbon in the presence of carbonic acid and steam.

The consequence of this is that the heating period in an oxidising atmosphere is quite useless, and one of the main features of the new process is to burn in reducin atmosphere from the beginning to the end, except finally a short terminal period in neutral or slightly oxidising atmosphere.

In the case of heating by burning a gas, the fumes contain amounts of carbonic acid and steam substantially over those strictly required for the conversion of carbon into gas. The removal of the carbon is then automatic and the amounts of carbon monoxide thus formed are not so great that they can have any real effect on the composition of the fumes.

On the other hand, it was granted up to nOW that the reduction of salts and metallic oxides contained in the paste called for a material amount of reducer gas in the fumes. In fact, the experiments undertaken by the applicant have proved that a minute percentage is enough a paste with a content of 3% iron oxide being reduced entirely in 9 hours by an atmosphere carrying 0.4% carbon monoxide and produced by burning town gas. In fact, it is enough then for the very low percentage of iron that characterises the paste used for porcelain and as a rule any ceramic paste, to have an atmosphere which is very slightly reducing and contains practically less than 1% of carbon monoxide and hydrogen.

This very low carbon monoxide content helps moreover to a large extent the conversion of carbon to gas and above all further checks the already minute chances of carbon deposition at low temperature, in the area corresponding to the breaking up of the carbon monoxide. This dissociation is given out in the chemical equation:

The lower the percentage of CO in relation to that of CO2 and the lower the temperature from which the dissociation starts and, consequently the slower will be the reaction and thereby, in a given period, the deposition of carbon. For this reason the atmosphere to be obtained has as its special feature a reduced percentage of carbon monoxide and a raised percentage of carbonic acid, accompanying increased steam contents being additionally of great interest.

These comments are equally important for the introduction of the process for furnaces performing the burning without combustion such as electric furnaces of all kinds or without contact between the products to be burnt and the combustion products or heating fluids.

In this case an artificial atmosphere is kept up in accordance with the process requirements throughout the length of the furnace effecting the heating, but there is a possibility of cutting out the arrangements such as fire doors, blowers or other accessories planned to keep the area at high temperature separate from that of preheating and in which the former practice was to have In brief, the improved process possesses outstanding points in its favour through the simplicity of the method of building the furnaces. The more or less complicated arrangements used for the avoidance of air in the high temperature area, for bringing back to the particular point where the coating temperature of glaze coating is obtained the hot air recovered in the cooling area or for removing in the preheating area all contact between the products to be baked and the fumes coming from the high temperature area, become quite useless. The troubles met with in regulating the furnace, especially those derived from the uneven balancing of the pressures are substantially lessened. The hot air recovered in the cooling area may be used completely at any rate for the partial heating of the drying kiln that is generally associated with the baking furnace.

Another main feature of the invention relates to the speed of baking and cooling.

The glazing of the hard porcelain paste is done at a temperature below 1200 C. and can be executed in less than four hours. Under such circumstances, in accordance with the new process, the complete baking of hard porcelain, with good surface covering of the enamel, takes less than nine hours.

What is more, apart from the problems of atmospheres, applicants investigations have proved that the cooling of the porcelain could be done very quickly. Thus the porcelain baked at 1400" C. may be cooled at the rate of 700 C. per hour between 1400" C. and 700C. On theother hand, for the quality of the enamel, as much for its colouring as for its surface appearance, it is far. better to carry out a very slow rate of cooling for the first 50 C. of cooling, after which it can proceed very quickly. The baking curve that is obtained from carrying out the new process in The arrangements featured are presented only as examples: all the details of execution and undertaking may be modified in every case without widening the scope of the invention as defined in accompanying claims.

What I claim is:

1; The method of firing enamelled porcelain consisting in firing the porcelain from the beginning until near the end of the firing operation in a reducing atmosphere including a smal1 percentage of carbon monoxide.

2. The method of firing enamelled porcelain consisting in firing the porcelain from the beginning until near the end of the firing operation in a reducing atmosphere including a small percentage of carbon monoxide and a higher percentage of carbon dioxide and of steam.

condition, the curve may for a given baking re- 7 sult in the minimum fuel consumption.

The substantial shortening of the baking time is a favourable feature of the process, but an increase of the time beyond the necessary minimum does not introduce any change.

The practice of the new process when the baking time is greatly shortened calls for a close contact between the piece andthe furnace atmosphere. The saggars or boxes of refractory material containing the pieces therefore must be amply notched or put together in extremely porous material. The packing of the pieces as used at present in round kilns would not allow a suitable performance of the new process. In fact, as a result of the low porosity of these boxes of refractory material the porcelain bakes in the air enclosed in the box at the time of the stacking of the pieces while this air is not driven out or only very slowly by the inlet of the reducing atmosphere at the moment of the hardening of the paste. The pieces baked in these conditions, when less than six hours pass between the beginning of the baking and the reaching of the temperature of hardening of the paste, when viewed in transparence, are reddish, their surface is yellow and often has bubbles. Their enamel is waxy;

In accordance with the invention, open boxes are employed as explained for ensuring a close contact between the baked pieces'and the furnace atmosphere.

Another advantageous point of the invention is that the new process allows the baking of the furnace blue or Sevres blue in much better circumstances than at present exist. As is known, this blue is obtained by means of cobalt oxide. This blue changes in very highly reducing atmosphere. On the other hand, cobalt oxide volatilises in an oxidising atmosphere, starts to deposit on the refractory boxes, and spreads out into the furnace where it may be deposited on white pieces. In the new process, these two drawbacks are avoided and it is possible to bake pieces in blue at the same time and in the same furnace as white pieces. As the volatilization of the colouring agent is altogether cut out, the boxes that have had blue pieces may be used again for white ones.

The new process thus forms an innovation of high interest for the baking of furnace blue.

The baking of so-called sharp fire enamels may be carried out equally advantageously.

3. The method of firing enamelled porcelain consisting in firing the porcelain from the begin- I ning until near the end of the firing operation in a reducing atmosphere, for a period approaching but not exceeding nine hours.

4. The method of'firing enamelled porcelain consisting-in firing the porcelain from the beginning until near the end of the firing operation in a reducing atmosphere and then cooling the porcelain at a rate of 700 C. per hour between 1400 C. and 700 C.

5. The method of firing enamelled porcelain consisting in firing the porcelain from the beginning until near the end of the firing operation in a reducing atmosphere, then cooling the porcelain very slowly and then cooling the porcelain at a rate of 700 C. per hour between 1400 C. and 700 C.

6. The method of firing porcelain enamelled with furnace blue and the like sharp fire enamels consisting in firing the porcelain from the beginning until near the end of the firing operation in a slightly reducing atmosphere including less carbon monoxide than carbon dioxide and then heating the porcelain for a short time in a substantially neutral atmosphere.

7. The method of producing hard porcelain from semi-vitrified enamelled pieces which comprises directly firing said pieces from the beginning until near the end of the firing operation in a reducing atmosphere.

8. The method as set forth in claim 7 which comprises in addition, the step of heating the fired products for a short time in a neutral atmosphere.

9. The method as set forth in claim 7 which comprises in addition, the step of heating the fired products for a short time in an oxidizing atmosphere.

10. The method of producing hard porcelain from semi-vitrified enamelled pieces which comprises firing said pieces from the beginning until near the end of the firing operation in contact with a continually renewed reducing atmosphere.

LOUIS BEAUDET.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,577,124 Jackman Mar. 16, 1926 1,802,776 Pence Apr. 28, 19 1 1,968,052 Norton et a1. July 31, 1934 2,112,777 Hauser Mar. 29, 1938

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