The preparation of slips, glazes and enamels Is a simple operation, but is one requiring great accuracy and a mind perfectly free from other cares. It is very easy to make mistakes, and they are generally quite disastrous. With the number of materials that must be handled and the diverse and often odd weights that must be used, it is sometimes quite perplexing to one who is not in the habit of doing it Of course, constant use of certain formulae makes the operation of weighing out mixes largely mechanical, but often this is the source of greatest danger, as the mind is apt to wander to other matters, and a mistake is the result, so the old advice to keep your mind on your business comes in with full force.
The arrangement of mixing and grinding rooms will vary so for different kinds of work and for the quantity of work to be done by this department that any attempt to even indicate it in a work of this character would be hopeless.
There should be sufficient bins in the mixing room to hold all materials of which large quantities are used, and these bins should never be changed, so that the mixer can find any material, even in the dark, if necessary. I have found the most convenient arrangement to weigh in to be a shallow box, holding about two hundred pounds of felspar, with a piece of wood firmly fastened level with the top of each side, and projecting eight to ten inches beyond the ends. These pieces of wood will form two handles at each end, so that the box can be carried in the same manner as a stretcher. The scales used should all be arranged for the same kind of weights. I have frequently seen a large and medium sized pair of scales arranged for avoirdupois and a set of grain scales arranged for troy, or apothecaries' weights, and the operator had to spend much time in reducing one system to the other. Of course, this only applies to experimental work, as grain scales are not used in regular work. Pint measures and small vessels for experimental work are best to be of granite ironware of good quality. For dipping vessels I have failed to find anything better than a good wooden washtub.
Glazes should all be ground. Slips may be prepared either in a blunger or a grinding pan. The latter is preferable, even for slips. It is not so rapid or economical to work as the blunger, but does better work. The storage tanks or vats will depend upon the size of the business. If it is small, small, deep tubs holding, say, one hundred gallons, and that are low enough to be easily stirred by hand, are all that is necessary, but even in a small business large vats, containing a stirring device, kept constantly running, will be found advisable for white slip and white glaze; in a large business a number of such vats will be required. The stirring device should run very close to the bottom of vat, so as not to allow much accumulation under it
The charge of slip or glaze being weighed out, it should be fed slowly into the blunger or grinding pan, which has previously had a little less than the requisite amount of water put into it Charge the clay first, then the lighter non-plastio materials, then the heavy non-plastic materials. In case plas-ter of paris is used, it should either be charged first or mixed with water by hand and added. If the capacity of preparing apparatus is limited much time may be gained in preparation of slips by putting the clay to soak in a tub or barrel over night. The sieving may be done by hand, but even in small plants a mechanical device will pay. It is better to use two sieves, passing the material through a No. 60 or No. 80 brass wire sieve first, and then through a No. 100 brass wire sieve for slips, and through a No. 100 brass wire sieve and then a No. 150 for glazes. This saves the finer sieves, which are weaker and more expensive. The very best sieving arrangement known to me is a frame suspended from a shaft above and oscillated through about a two-inch or three-inch space very rapidly, I should judge about six hundred times per minute. The sieve is placed on a slight slant, and is entirely open on the low side. This sieve is entirely mechanical, the sieved material falling through it into a box below, from which it may be conveyed to its final destination. The grit and dirt gradually works to the low edge and drops off into a trough or box placed for its reception. There may be two of these sieve frames, and the material can pass directly from one to the other, or a single frame can be arranged so that the sieve can be easily and quickly changed, and the charge, as it la sieved the first time, can be elevated into a proper tank, and when the first sieving is finished it can be passed through the second sieve. After passing through the second sieve it can be elevated by pumps into the final receptacle. All slips and glazes should be resieved just before using. A No. 100 sieve is fine enough for this purpose, as the object is simply to break up any stringiness or lumpiness, and is usually best done by the dipper or his boy. If you use English ball clay, No. 4, and a vat system, allow your slips and glazes to become at least a month old before using. If you use the hand system and carry no stock, and for orders for special colors, use the slip or glaze immediately. In a few days a fermentation takes place in liquids containing this clay, and it fills with gas bubbles. After a month or so this fermentation ceases, and the slip or glaze becomes smoother and tougher than at first Where possible, carrying slip over the winter is a good thing. Allow it to freeze and thaw as much as possible, re-blunge and sieve, and a very smooth, fine slip will result. Where colored slips are made by staining white slip and colored glazes by staining white glazes, it is advisable to reduce such receipts to slop receipts, either adding so much dry stain to so many buckets, gallons or pints of slip or glaze, with the slip or glaze of a fixed weight per pint, or adding a certain quantity of slop stain, at a fixed weight, to a certain quantity of slip or glaze, at a fixed weight This enables one to use old slips and glazes even for colors, without the necessity of carrying a stock of these colors.
As this is written more for those who are experimenting in this business than for those who understand it a brief description of the easiest way to prepare slips and glazes by hand will be in order. In receipts where English ball clay, No. 4, is used, much labor is necessary to reduce it to the proper degree of fineness, as it is a very gummy, tough clay, and contains considerable vegetable matter. Grind or break the clay fine. Always put water into the mixing vessel first and add the clay to the water; never pour the water on the clay.
For slips you need a No. 40, a No. 60 or 80 and a No. 100 sieve; for glazes, a No. 60, a No. 100 and a No. 180 or 160 sieve, all of brass. Never use a silk lawn except for stains. After the materials are all thoroughly soaked and mixed with the hand and worked as fine as possible, for slips, rub the mixture through a No. 40 sieve, using a block of hard wood to rub with. Rub until every particle of clay is through. Then rub through the No. 60 or No. 80, then through the No. 100. After it has been rubbed through the No. 100 sieve, shake it through the same sieve, and it is finished. For glazes, treat in the same way, substituting the 60 sieve for the 40, the 100 for the 60 or 80, and the 130 or 150 for the 100. The slip or glaze should be thicker than is needed, when you commence, as the rinsing of vessels, etc., will thin it considerably. The thickness of a slip can be much affected by acids and alkalis, acids usually thickening and alkalis thinning. This may be the reason that aging makes such a difference in the thickness of some slips. Chemical combinations may take place that alter its acidity or alkalinity. In sieving, always have the mixture to be sieved as thick as possible. The material becomes much cleaner without using such fine sieves. There is now being made a No. 200 brass wire sieve cloth which makes a very good substitute for the No. 16 silk lawn. If soluble fluxes are used in either glaze or slip, these glazes or slips must be mixed of regular thickness, as water cannot be taken off without removing some of the flux dissolved in it.
The thickness to which slips and glazes should be mixed depends upon the condition of the clay to which they are to be applied, and can only be learned by experience. The weight per pint will vary, depending upon the character of the materials used. The more tough clay the lighter the mixture will weigh for a given thickness, and the more heavy material (such as barytes, lead, etc) the heavier it must weigh.
All stains and frits should be ground exceedingly fine, certainly fine enough to pass easily through a No. 150 sieve, and it is better to have them pass through a No. 200 wire sieve or a No. 15 or No. 16 silk lawn. There are a number of machines for the grinding of these materials. There is a modification of the Alsing mill, a porcelain-lined rumble, carrying flint pebbles. These machines grind wet or dry, but are expensive. They are rapid grinders, and their dry grinding is of great value. They are easily cleaned to change from one color to another, and where the stains are carried in stock dry the use of this machine obviates the necessity of drying the material after it is ground. In the wet mills there are a number of different styles, some high-priced, quick-grinding machines, others low in price, but slow grinders. As a rule, a number of the slow, cheap machines will be found better than a few of the high-priced, rapid grinders. There will be less changing of colors in each mill. The thorough cleaning of a wet mill is a great nuisance. The mills driven from overhead are preferable to the undergeared mills. I have had experience with both, and my opinion is very decided in this particular. With a plant of overgeared mills we rarely, if ever, had any trouble, and with a plant of undergeared mills, made by the same maker, we were nearly always in trouble.
The pumping machinery for slips and glazes can be of the simplest character. Common cucumber pumps, arranged to drive by power, will be found to answer every purpose for elevating slips and glazes. They are very cheap and will give good service. One of them will handle slip enough for at least sixty million brick before it is worn out. In a small brick plant the pump would probably rot out before this quantity of slip would be used. The little calculation was made from a cucumber pump in a pottery that handled five to six tons of clay in slip form every day, and would wear from two to three years.
It is probably needless to say that all slip and glaze vats should be covered, and that every precaution should be taken to keep all prepared material scrupulously clean.