During the past five years there has been much written in the Clay-Worker, and somewhat in other papers, on this subject There has also been one English work published that is devoted entirely to it. It is very interesting to review all this matter and note the diverse ideas of different writers and the inconsistencies of some of them. I want to call attention to some of the marked contrasts and inconsistencies, and show therefrom that there must be many roads to success in this business, unless the writings referred to are untruths.
One writer condemns emphatically the use of lead, potash, nitre and all low-heat fluxes in brick glazes, and also condemns and ridicules the use of a frit, then, in giving the materials that should be used in glaze, mentions flint glass. Flint glass is composed of flint, lead, potash and nitre in varying proportions. An average receipt would be as follows:
Cullet (old scraps)
.......60 to 100
So we see, in advising the use of flint glass, this writer advises the use of lead, potash and nitre, and yet, in another place, condemns the use of the same. In advising the use of flint glass he also advises the use of what is nothing but a frit, and, unfortunately, a frit the exact composition of which is unknown to the user, and which he would have no guarantee of being able to duplicate. Is it not much better to make the flint glass for one's self in the shape of a frit? The proportions can then be varied to exactly suit the requirements.
Again, glazing dry only is recommended by some and glazing wet only is recommended by other writers. Molding hand-molded brick in water is advised by some and in oil by others. I think there is very little doubt that if the clay can be molded easily by the slop method it is much better than to use oil. The less oil that is used on a brick to be enameled the better.
One of the most amusing statements I met with was to the effect that a white enamel was more uniform in color than a white made by the slip and glaze method. We all know that a white enamel is only translucent, while a white slip is absolutely opaque. A white enamel is apt to flow to some extent, while a white slip is immovable. A white enamel becomes more thoroughly fused as the heat raises. A proper white slip is very slightly, if at all, affected by changes of heat; that is, so far as color and opacity are concerned. Yet, notwithstanding all these facts, the above statement was made, which amounts to saying that a translucent material is a better hider than an opaque one, a flowing material more apt to stay of even thickness than an immovable one, and a material easily affected by difference in heat is less apt to be affected than one less easily affected. In every case the reasoning seems to me to be just hindside foremost. I do not wish to be understood as condemning the enamel process -hardly! - as I frequently use it myself; but it is foolish to claim as its good qualities just those in which it is most deficient Its good quality, in my opinion, is its perfect adhesion in low-heat work; that is, it is possible to obtain a good white on buff clay, by using an enamel, without the use of a slip, and as the enamel is a fused glass it adheres perfectly to the brick body. Slips are sometimes difficult to attach thoroughly to low-heat bodies.
One writer actually insinuates that another must be telling an untruth, or that he has not expressed his meaning, when he states that enameling proper can be done on green clay, and further ventures the assertion that it cannot be done; also ventures the assertion that an enamel cannot be made without previous fritting. A third writer says that enameling proper can be done on the green brick, but does not advise any one to try glazing on the green. Now, what are the facts? First, an enamel proper can be made either wholly of frit, partially of frit and partially of raw material, or wholly of raw material. A glaze can be made in the same way, and either enamel or glaze, when made either partially of frit and partially of raw material, or wholly of raw material, can be successfully and easily applied to green clay. Either enamels or glazes can be made to burn at any degree of heat, I believe, although I will not be certain of the enamel at the extreme high heat of the porcelain kiln, as I have never tried it at that heat, but I have used it with success to very nearly porcelain heat
Again, a writer condemns the use of oxide of zinc when working on green clay, while others advise its use and claim that it improves glazes. All I can say is that I have never noticed any bad effects from oxide of zinc used in reasonable quantities, but, on the contrary, have found it to be a good whitening and brightening material in a glaze. I really believe most of the zinc is volatilized before the firing is completed, but in its volatilization it seems to carry off deleterious matter; possibly the oxygen given off from it during the burning has a beneficial effect. Oxide of zinc used in large and injudicious quantities tends to cause a slip or glaze to crack.
Another statement met with is that there is less liability to craze when glaze is put on thin than when it is put on thick, and then a thickness of three-thirty-seconds to one-eighth of an inch is mentioned as the proper thickness.
The first part of this statement is undoubtedly true of some bodies, but the exact opposite statement is just as true of other bodies. On vitrified or hard bodies, as a rule, the thinner the glaze the better, while on soft, porous bodies often the thicker the glaze or enamel the better. As to a thickness of three-thirty-seconds to one-eighth inch, I should consider such a thickness altogether too great and unnecessary. From one-thirty-second to one-sixteenth inch thick 1 have always found ample. One-eighth inch of enamel on an English size brick would be about one-thirty-fifth part of the whole. The enamel would certainly have a higher specific gravity than the brick, so its weight would be not less than one-fourth of a pound, or two hundred and fifty pounds to the thousand brick. Take the low figure of 4 cents per pound mentioned by this writer and we have a cost of $10 per 1,000 brick for the item of enamel alone. Many enamels cost nearer 10 cents per pound; in that case the cost would be $25 per 1,000 brick for this one item. Bricks can be slipped and glazed for $2 per 1,000 easily - that is, for cost of material - and I have done it for 90 cents per 1,000.
The writer also stated in an article in the Clay-Worker that "my experience has led me to believe that only high-heat clays are adapted to the single-fire method." I wrote quite strongly on the subject, but now wish to state that further experience leads me to retract the whole statement, and to state, instead, that my experience leads me to believe that there is a large range of heat through which clays can be adapted to the single-fire method.
The conclusion to be drawn from all these differences of opinion is that there are many different methods and ways of making enameled brick, and that none of us knows anywhere near all of them.