plaster head on which the moulds are placed. The presser keeps this in motion with his left hand, whilst with the right he guides the polisher.
In those manufactories which have adopted the latest improvements, the jiggers are worked by steam power, and the stoves in which the pieces are sent to dry are heated by steam pipes. These are constructed on a new principle, consisting of a number of shelves which revolve round a central spindle, so that by a gentle push of the hand, each section is successively brought in front of the door, giving the opportunity of removing or putting in the moulds. This simple contrivance does away with the necessity for the assistant boy entering the stove, and feeling the bad effects of the heat.
When the pieces are not exactly round, and cannot be thrown or pressed on jiggers, it is the custom to have them made in plaster moulds, which have been cast on models prepared for the purpose. As long as the clay keeps soft, it takes the shape of any hard substance against which it is pressed, and for that reason, plaster, which has the property of absorbing moisture readily, is preferred. The use of plaster for moulds is comparatively recent, and although its properties were known in early times, there is no evidence that it was ever employed for that object. Greeks, Etruscans, and Romans, had their moulds made of fired clay; the Chinese, in raw clay thoroughly dried. In Staffordshire, before the use of plaster, they were made of fired clay or metal; but plaster is more economical than any of these, although moulds made of this material do not last long, and require constant renewing.
The making of moulds, well adapted for pressing the various shapes, is a very important part of the potter's business. They must allow of a certain amount of contraction, and, at the same time, must easily dislocate without pulling away any part of the piece, which is still sufficiently soft to be distorted by careless handling. Some pieces will require moulds made in one or two parts; others, a large quantity of them, the various fragments being in that case pressed separately, and carefully put together afterwards. The pressing is done in this way: the potter begins to flatten a lump of clay in the form of a bat, and transfers it to the inside of the mould; then, by the repeated blows of a sponge in his right hand, he compels the soft material to take the exact form of the mould, and, of course, of any ornamentation which may be on its inner surface. A good presser ought to be systematic in his work, and not to apply more pressure to one part than to another, otherwise the different portions of the pieces would not contract alike, and would be liable to show an irregular surface, or even crack in the drying or firing processes.
For several reasons, there are pieces which cannot be pressed: they may be required very thin, or their shape is such, that the potter cannot reach all the parts to take the impression conveniently. In this case he must adopt the following plan. The mould is tied up, and filled with liquid clay through an opening left in the top. The plaster rapidly absorbs the water, and a deposit of solid clay adheres to the surface. This soon increases in thickness; and when the potter thinks it is sufficient, he pours out the slip which is in excess. The piece soon hardens, and when it begins to contract, it is then time to remove it from the mould. This process has the advantage of giving a uniform thickness, and as there is no other pressure than that caused by the absorption of the plaster surface, there is a better chance for the piece to contract equally, and on this account this method (called casting) is preferred for articles which require a neat execution. In some cases it is cheaper than ordinary pressing; but the drawback is, the excessive contraction or diminution of bulk to which the ware thus made is subjected. An irregular contraction is the source of most of the defects attending the ceramic manufacture, and it is worth explaining the causes, of which there are three. I have already mentioned that natural clays, which have remained in a damp soil for ages, contain materials in a hydrous state, i.e. combined with water, which sometimes increases their bulk considerably. These are unstable compounds, and may be destroyed by thoroughly drying them. Some other materials used in pottery may be artificially combined with water, as would be the case, if ground in it for an unnecessary length of time. The second reason is, the interposition of the uncombined water between the solid particles of the clay, and as this cannot be worked without it, this cause of shrinking cannot be avoided. It will be easily understood, that when the water in the mixture evaporates, the solid particles, under atmospheric pressure, will move to take its place, and this effect will continue as long as they find enough moisture to assist in their free motion. The consequence is, that the mass shrinks more and more, till the contraction is stopped by the inability of the particles to move farther; and this happens before the pieces are completely dry. From that state to complete dryness, the evaporation of the remaining water will leave small holes, which will make the texture of the ware porous, and prone to absorb any liquid with which it may come in contact.
The shrinkage in the raw state then is mechanical, and distinct from that which takes place in the oven under the influence of heat. Under this agency the particles enter into combination, and if the process is carried far enough, the ware may become partially vitrified and acquire a certain amount of transparency. The more perfect the vitrification, the closer will be the contact of the particles, and consequently the greater the diminution of bulk. From these causes, the total contraction may vary from one-sixteenth to one-fifth of the original model. The least will belong to ware pressed with stiff clay gently fired; the greatest, to that cast with liquid slip and brought to the vitrified state. In these last, the shrinkage is greater in height than in width, a fact explained by the weight of the upper portions acting vertically to assist the closer contact of the particles in the under-structure, when the same opposes their free action in an horizontal direction. In making the models, care should be taken to bring the contraction to a common centre, or if there are several, to strengthen sufficiently the connecting parts.
After the drying of the ware, the next operation consists in placing it in saggers, which, as I have said, are made of common fire-clay, and of a form and size to suit the different articles which they are intended to hold. A certain thickness of flint or sand is placed at their bottom for the purpose of giving them a firm bed, and as it is the interest of the manufacturer to make the same firing answer for the greatest quantity of goods, care is taken to fill the saggers as far as is safe. The placing of the ware is done at the outside of the ovens, and when these are to be filled, the saggers are quickly arranged one over the other in columns, called "bungs," each sagger forming the cover for the one immediately underneath. A small roll of soft clay placed between makes them stand better, and at the same time prevents the ashes carried by the draught from finding their way into the interior, and damaging the contents.
In ancient times, the ovens, intended to hold few pieces, were very small; but as the potters became more experienced, the sizes were gradually increased, and now-a-days some of them are not less than 19 feet in diameter. The quality of fuel had, of course, a great deal to do with their mode of construction. Now, however, that coals are acknowledged to contain more heat, and to be cheaper than wood, the ovens are generally built in a cylindrical form, with several mouths or feeders disposed at equal distances on the outer circumference, the upper part being covered by a semi-spherical dome or vault, to keep the heat inside and reverberate it downwards. This construction is very simple, the only complication being in the arrangement of flues under the bottom of the oven, so as to throw into that part a portion of the heat, which otherwise would be liable to accumulate towards the top.
The firing must be conducted very slowly at first, to prevent a too sudden evaporation of the damp, which would cause the splitting of the goods. This being done, the heat is raised gradually, care being taken to feed the mouths with fuel as quickly as it is consumed. It requires an experienced fireman, to see that one part of the oven does not get in advance of the other. He manages this by throwing in a certain quantity of air through small openings in the brick-work, which are shut or left open according to circumstances. Whatever may be the construction of the oven, the quantity of air mixed with the gas produced by the combustion of fuel causes the atmosphere to be reductive of oxidizing; which means that the different materials submitted to the heat would, in consequence of an abundance of carbon, have a tendency to be deprived of their oxygen and return to a metallic state, or that by firing in presence of an excess of air or carbonic acid, they would be kept in a high state of oxidation. It is fortunate that all classes of English pottery, without exception, require, or are not injured by,
