It has previously been set forth that the presence of materials in the glaze such as are not taken up by glasses on fusion can hinder the production of a faultless glaze, because on the one hand, the temperature of glaze formation is never so high, as to permit the impurities to separate out as glass gall, and on the other hand, the thinness of the layer prevents materially the occurrence of such separations.
Among those materials which are capable of giving trouble in this connection, because they are either not absorbed at all, or not sufficiently absorbed by glasses, the sulphates and chlorides of the metals play an important part. Among the impurities which exist in the materials used in glazes, are those salts which contain a volatile acid, like carbonates, acetates, nitrates, etc., which are decomposed, those which contain non-volatile acids, like phosphoric acid which melt without influencing the brilliance of the glaze, together with those which like the sulphates and chlorides offer a greater resistance to absorption by the glass.
The sulphates, at least so the text-books say, are completely decomposed by silica, but this does not happen, at least not at those temperatures which come into consideration in the melting of glazes, and the other requirements which would cause a ready decomposition of the sulphates are never present. Chlorides are only decomposed in the presence of steam and then only at very high temperatures; common salt can be removed from the glaze or frit by sufficient leaching, but the sulphates generally form insoluble salts which cannot be leached out, like the sulphates of barium, lime and lead.
It is now well known from the researches of Pelouze on the making of glass, that sulphates can be dissolved in a glass without suffering decomposition thereby, and the content of sodium sulphate (which most resists decomposition) may even amount to 3 per cent of the whole weight of the glass. Such sulphate glasses have the property of efflorescing. These glasses, soon after exposure to the air, show a white deposit which can be easily washed off, but appears again in a few days, and spoils the surface of the glass. Very similar coatings can be seen on glazes which points to the fact that sulphates separate out from the glass, and if wiped away, appear again. Those glazes which contain potash show this property the most, and those which contain soda, the least.
A second and much more unpleasant manifestation occurring in glazes containing sulphuric acid, consist of a complete obscuring of the surface of the glaze, and a generation of bubbles in the glaze. Sulphates are less easily soluble in the more acid silicates than in the less acid ones, and since the glazes tend to take up silica from the body of the ware on which they are melted, and therefore to become more acid, it may easily occur that a part of the contained sulphates is expelled.
This is always accompanied by the generation of a gas, and remains noticeable in the glaze, because the layer of glass gall which forms simultaneously on the surface of the glaze, prevents the bursting of the bubbles. A coating thus forms on the surface of the glaze which, like the cream 'on milk, hinders the bursting of the bubbles and makes the glaze appear frothy. If one were to melt in the same fire the two following glass batches, BaO, SO3 + 2Na2O, CO2 + 3SiO2 -f B2O3 , and BaO, SO3 + 2Na2O, CO2 + sSiO2 + B2O3 , he would obtain two layers in the crucible. The lower layer would consist of an excellent clear glass, on which floats a crystalline streaky material, which dissolves in water, leaving a residue of silica. This upper layer consists mainly of sodium sulphate so that the decomposition must take place according to the reaction BaO, SO3 + 2Na2O, CO2 + n SiO2 = BaO, Na2O, n SiO2 -f Na2O, SO3 + 2CO2 .
The clear glass which is found under the cover of the sodium sulphate must be saturated, at the temperature at which it has been formed, with the sulphate. In fact, the first contained 4.50 per cent, and the second 2.10 per cent, of sulphuric acid. The first corresponds in its composition to a bisilicate, and the second to a trisilicate; both when melted on shards of ware, show not a glassy surface, but form a frothy mass, though they were previously melted perfectly clear.
The deduction from this experiment is that on the one hand the solubility of the sulphates decreases with the acidity of the glaze, and on the other hand that an expulsion of the sulphates takes place if the glaze has an opportunity to absorb silica. If we look around for the sources of these sulphates, we will find them readily enough, and even in the body of the ware itself. Clays often contain sulphates, which are soluble in water, and which effloresce on the corners and projections of the ware, but are made insoluble by the burning process.
Frequently the presence of sulphates in the clay can be detected by the colors imparted during the biscuit burn. Often when they are white they are not visible to the naked eye, but on examining them with a glass, they appear as little wartlike prominences. A second source of sulphates lies in the water which is used in making up the glaze. Hard water, and especially such holding calcium sulphate in solution, ought not to be used for these' purposes. A third source of sulphates we find in the glaze materials themselves. The soda, borax, boracic acid, potash, and niter, intended for the frit fusion, often contain noticeable quantities of sulphates, and on this account ought to be carefully tested.
The more sulphates a glaze contains, or can absorb from the body, water, or glaze ingredients, the more it will be in clined to generate gases in the fire. We can in fact explain the evolution of gas in a glaze in no other way, than that the glaze contains some material which, under some conditions, will remain in combination, and under others will be set free and escape. In sulphuric acid we recognize such a material. It can occur in a glaze up to a certain amount, without affecting the brilliance of the glaze; but at a higher temperature, and when the absorption of silica begins, it separates from the glaze and forms a delicate skin on its surface, and hinders the bursting of the bubbles as they rise, formed simultaneously by the generation of sulphuric acid, or sulphurous acid, or oxygen.
The defects which a content of sulphates causes in a glaze are as follows:
It forms first a skin on the surface of the glaze, which often has a wrinkled appearance; this skin forms earlier on spots where the glaze is thin ( and therefore has taken on an acid nature from the absorption of silica from the body) than where it is thick. Under this skin little bubbles assemble which do not burst, but which afterward break as soon as the glaze is touched by the hand. At the same time they will cut off, with their sharp edges, portions of the skin, and now appear no more as bubbles, but as black points or pits in the surface. Large bubbles form only where the glaze is thick. The phenomena, which are associated with the drying up of the glaze on the edge of plates, is probably due to sulphates in the glaze. As a rule, the glaze not only is thinner at the edge than elsewhere, but also the quantity of sulphates which have arisen from the body is generally greater there. The glaze has by no means been absorbed by the ware here more than elsewhere, and the piece looks dull, because there is on the surface of the glaze a layer of "gall," which makes it appear totally lacking in luster. Sulphates do not always leave their traces in the glaze, in fact, only in the presence of oxidizing conditions of the kiln atmosphere; only then do these efflorescences occur which have been described. If the melting of the glaze is accompanied by a periodically reducing atmosphere, a reduction of the sulphates to sulphurous acid occurs readily, accompanied by the volatilization of the latter. These phenomena, which are connected with the presence of sulphates in the glaze, do not appear when, during the melting of the glaze, a sufficiently reducing atmosphere has predominated in the kiln. From this property of the sulphate salts, it is easily understood why, in the porcelain industry, it is possible to use gypsum itself as a glaze ingredient as is done in many places.
Porcelain, when it is burnt under normal conditions, is burnt in a reducing fire. In this the sulphuric acid is completely volatilized at a temperature at which the glaze has not come into complete fusion, provided, the reducing fire is continued with sufficient length and intensity; the glaze under such conditions flows out smoothly. But if the reducing flame does not develop with sufficient intensity, sulphuric acid will remain in the glaze, which is liberated gradually with the rising temperature, and produces the phenomena called "trichinae" or "scabby," that is, the glaze forms bubbles which do not burst in the fire, but subsequently when stroked with the hand, and become dark-colored by the filling up of the cavity with dirt.