Natural hydrated sulphate of lime. This is plaster of paris before it has been burned and made into plaster.
There are two materials which are used under this name, one the sulphate, the other the carbonate of barium. They are both fluxes at a high heat, and tend to make a glaze still more complex.
Fluoride of lime. A lime flux, acting at a somewhat lower heat than the carbonate or sulphate of lime.
A double fluoride of aluminium and soda. A very good, extremely soft flux, insoluble in water, not to be used in large quantities.
- Oxide of Zinc This is a flux, and, at the same time, a whitening and brightening material in the glaze.
Carbonate of lead, or white lead; oxide of lead, or red lead; litharge, borate of lead, aluminates of potash and soda, borate of lime
Salts of soda and potash.
This is a large number of materials with which to work, and almost any conditions can be made to exist by their proper combination.
Now, what is enameling? As usually employed, it is to coat a material of one kind and color with a material of another kind and color that shall have a smooth, glossy face. In order that enameling may be successful the coating must adhere perfectly to the material to be coated.
The natural adhesion in slips is the adhesion of clay to day. This may be enhanced by the addition of fluxes. In enamels we must rely upon the fluxes almost entirely. In order that the adhesion may be perfect, the body and coating must contract in such relative proportions as not to break the bond between them, for this bond, once broken, when the natural adhesion of clay to clay is relied on, cannot be restored, and where the adhesion produced by fluxes is relied on it cannot be restored to its fullest extent. The slip or enamel should shrink more than the body, not to an extent to produce cracks, but still, to some slight extent, its shrinkage should be greater than the body. If it shrinks less than the body it will surely shell off, and so there should be a margin of safety, for even should the bond between the body and coating not be broken to such an extent as to cause immediate shelling, it may cause an unreliable article. It is very peculiar that, in drying, the slip will not stand the least compressive strain without shelling, but no amount of tensile strain seems to tend to produce this effect
In work that is slipped we may have adhesion to the fullest extent the slip is capable of, but if it is a soft burning slip at the heat used, its capabilities in this direction will be small. Such a slip may not chip or shell off, but the glaze over it once broken through it can easily be scraped off. We therefore must have a slip that not only adheres well, but that will burn hard at the heat used.
The shrinkage can be controlled by the amount of ball clay or other high shrinking clay used in the slip, and the hardness of the burned slip by the amount of flux it contains. In high heat work or work where the heat is beyond the melting point of feldspar this is undoubtedly the best flux to use, but in heats below this some other flux must be used. I will not go into the relative merits of the different fluxes at this point, but will take it up later.
As a starting point we will assume a mixture as follows:
A good, tough ball clay.....................40
No. 7 English China Clay....................30
Our imaginary brick is dipped into this slip immediately after it comes from the press and put away to dry. The slip will probably either crack, shell or be all right In the first event we have too much shrinkage, and should replace some of the ball clay with either china clay or flint; in the second event we should reverse the operation, and in the third event we should put the brick into the kiln and burn it When the brick is taken from kiln there are nine different conditions that may exist It may be cracked, peeled, or all right in these respects, but too hard - that is, vitrified - and body may show through. It may be cracked, peeled or solid, but too soft It may be cracked, peeled, or solid and of the proper hardness.
If cracked and too hard, addition of flint will probably remedy both troubles. If addition of flint stops the cracking, but still the slip burns too hard, replace a little of the feldspar with flint
If peeled and too hard, it shows that shrinkage has not been great enough, so replace some of the feldspar with ball clay.
If this stops the peeling, but the slip still burns too hard, replace a further quantity of feldspar with flint If the first replacing of feldspar with ball clay brings to proper degree of hardness, but does not stop the peeling, replace some of the flint with ball clay.
If solid, but too hard, replace a portion of the feldspar with a mixture of ball clay and flint, in the proportion of two of ball clay to one of flint, or in the same proportions as they exist in the original mixture.
If cracked, but too soft, add feldspar. If this does not make the slip hard enough, resort to cryolite, trying different percentages from one to ten. It is seldom advisable to go beyond 10 per cent of cryolite. If cracking still continues replace some of the ball clay with flint.
If peeled, but too soft, replace some of the flint with ball clay. Continue this until peeling stops. If slip remains too soft, add cryolite.
If solid, but soft, try adding feldspar. If feldspar will not harden slip, add cryolite.
If proper hardness, but cracked, replace some of the ball clay with mixture of flint and spar, two of flint to one of spar, or as they are proportioned in the original mixture.
If proper hardness, but peeled, add ball clay, or add ball clay and remove a little of the spar.
If proper hardness and solid, the proportions of the different ingredients are correct
Any additions or changes that are here suggested will lead either to a continuance of the same condition in a less marked degree or else to one of the other conditions, so that these nine conditions and their probable remedies will lead out of all difficulty. The only failure that will be met with will be when the heat is so low that 10 per cent of cryolite will not make the slip burn hard enough. In that case make a flux by burning together in kiln a mixture of equal parts of borax and feldspar. Grind this very fine and use it instead of feldspar, but when this has to be resorted to the heat is probably too low to admit of a reliable glaze being used.
We are now supposed to have a slip that adheres well and is of the proper hardness, but nothing has been said about its color. The color of the original slip given would probably be a cream white. Staining with blue stain would give it a whiter appearance, but, at the same time, possibly a slightly greenish tint If better color is desired, it might be obtained by using more grit in body, reducing the shrinkage. This would admit of less ball clay in slip, or bricks might be slipped after they are thoroughly dry. In this case, much less ball clay, possibly none, would be used, and unless the heat is high an artificial flux might have to be resorted to. Much can be done to improve the color by using a good white enamel or semi-enamel over the slip. With heavy shrinking, low heat clays, a good white slip is difficult to obtain, and semi-enamels must be used.
Slipping dry clay I have found the most difficult to do. A slip with much plastic material in it cannot be used, as it will crack unless the bricks are dipped in very thin slip, allowed to dry and dipped again, repeating this operation several times, or until the coat is thick enough. This multiple dipping is expensive, and is apt to give some trouble by blistering. A short slip on a dry brick is almost certain to pin-hole badly, so probably the best way to work, where the heat is low and the brick body is such that a considerable amount of ball clay must be used in slip, is to slip the body while soft with such ball clay slip. Then glaze with an enamel or semi-enamel, or allow the brick to dry and dip it again in thin white slip, then glaze. The white slip for the dipping after the brick is dry will have to be varied to suit the heat I have always found ball clay and flint to work better than a china clay. The ball clay should not amount to more than 10 per cent of the whole, the balance to be made up of flint and spar, with possibly 4 per cent to 5 per cent oxide of zinc. If spar will not make the slip burn hard enough at the heat used, substitute the artificial mix of spar and borax mentioned above. Should this slip not hide the off-colored slip under it from 5 per cent to 10 per cent oxide of tin will help it very much. Lime will not help much to whiten low heat slips, but will help to whiten high heat slips. The pure white brick are the only ones that this trouble will have to be taken with. The off-colored white will stain for colors just about as well as a pure white.
The glaze should be very different in the percentage of clay it contains for wet dipping than for dry dipping. The composition of the glaze can be given a little more accurately for dry work than for wet In dry dipping it should contain about 5 to 7 per cent of a moderately tough china clay. For wet dipping it should contain from 10 to 20 per cent of a tough ball clay that burns a good color. From 11 to 13 per cent will probably hit the majority of cases. Soluble fluxes should, as a rule, be avoided in the glaze, especially for wet dipping, as they soak into the body, and, in most cases, cause the face that has been dipped to shrink so much more than the balance of the brick that it cracks. Where the glazing is done on the dry brick soluble fluxes do not seem to have this objection to such an extent Where the clay will stand it without cracking, soluble fluxes are excellent as they help bind body and slip together.
The make-up of a glaze is dependent entirely upon the heat, or, in other words, is dependent upon where you wish it to melt The clay, for reasons given above, may vary from 5 per cent to 20 per cent of the whole. I prefer the glazes with more clay in them when it is possible to use them. From 4 to 8 per cent of lime should be used, though the lime may run much higher. If little clay is used considerable feldspar should be used, so as to get in sufficient alumina. These three things will make a glaze, and a good one, but it will take a high heat to melt it A study of the receipts which follow, together with reference to the description of materials and the object of each, will enable the reader to figure out the law of glazes.