this is the case, the glaze will have to be touched up' before the goods are placed in the kiln so that these tendencies may be counteracted. In some cases very pretty effects are produced by allowing the glaze to run in certain directions. As a general rule, when ware is dipped it is best to have the glaze finely ground, as it dries more slowly in this condition, and so allows a more even distribution of the glaze on the surface of the goods.

Stripping off of the glaze or engobe is due to these not having a similar contractibility to the body of the ware. It is a special difficulty in the case of white enamels which contain tin as an opacifying medium, particularly when this substance is present in large proportions, e.g. over 20 per cent. Being due to the oxide of tin itself, this defect can only be remedied by a series of extremely careful experiments in altering the composition of the enamel without losing its opacity and gloss.

When stripping is due to the incompatibility of the body and engobe or glaze, it may generally be remedied in the way described under 'Peeling.'

Stunting. See 'Cracks.'

Sulphates, whether in the clay or in the water used, are apt to cause the formation of an efflorescence or scum on the goods. This is best prevented by the use of baryta compounds or by covering the goods with an engobe (see Efflorescence' and 'Scum ').

6

Sulphates are also apt to cause turbidity in the glaze, as they may, under certain conditions, liberate free silica. By the use of a reducing fire at an early stage of the firing the sulphates will be decomposed and their turbidity-producing power destroyed.

Sulphur occurs to a notable extent in poor coal and produces a characteristic effect on glaze, especially when the latter is coloured. This effect can only occur when there is insufficient air in the kilns and fires, as, except in the case of the very worst fuels, the sulphur dioxide produced from the fuel should be carried off before it can affect the goods. As it does not usually pay to attempt to purify the fuel, care should be taken that the coal used is reasonably free from sulphur compounds.

Sweating or running of the glaze is referred to under this latter heading.

Swelling of the glaze is often noticed when a kiln has been seriously under-fired, so that the glaze has just begun to melt but still remains porous and dull; on further heating to the necessary temperature the swelling will disappear and a normal result will be obtained, provided that the glaze has not lost too much of its more volatile constituents by prolonged exposure to heat.

Swelling of unglazed goods is usually due to their having been placed in the kiln in a very damp condition, and then having been so rapidly heated that the moisture could not escape without the exercise of considerable force.

Temperature being of the highest importance in the firing of the kilns, as well as in the drying of the goods, great care should be taken in regard to its measurement and regulation, as errors in the temperature of the goods at various stages of the manufacture may prove to be a source of enormous loss.

Thinness of the goods themselves is a matter for which the designer and maker are responsible. As goods alter in shape in the kilns in proportion to their thinness, care is required to adjust this to the purposes for which the articles are required.

Thinness in the glaze on the surface of the goods drawn from the kiln is due to an insufficiently thick coating of glaze. This may be caused by the glaze slip being too watery, to its having settled somewhat, to its not having been properly stirred up before use, or to the dipping having been done too rapidly. When other methods of applying the glaze are in use, the defect will be found in the majority of cases to be due to an insufficient coating of glaze material on the goods.

It occasionally happens that if a piece of glazed ware is placed too near to an unglazed surface in the kiln that the latter will 'rob' the former without getting glossy itself. The cause of this curious action is by no means fully understood.

Turbidity in a glaze is most frequently the result of underfiring, the glaze not being properly fused. Sometimes the accidental admixture of some infusible matter, particularly clay slip, will be the cause, and, less frequently, an error in the proportions of the different materials used in the compounding of the glaze. When the defect is very pronounced a curious egg-shell appearance is produced which, on re-firing at a somewhat higher temperature, will be converted into an ordinary transparent glaze.

Minute bubbles of air or gas which have not been able to escape, owing to insufficient time being allowed for the glaze to mature, are occasionally observed, particularly when the glaze contains compounds which evolve oxygen at high temperatures.

Prolonged heating of the glaze at temperatures below its meltingpoint is apt to produce turbidity owing to some of the alkali (flux) in the glaze being volatilised. This may be remedied by a more rapid heating at the earlier stages, or by the addition of as much more alkali to the glaze before use as will counteract that lost in the kiln; great care is, however, necessary, as too much alkali

will make the glaze 'trickle' or 'run' and will often be the cause of its crazing.

When the turbidity of a glaze is due to sulphur compounds the quality of the coal used needs attention; but even with good coal and a highly reducing atmosphere, insufficient air supply to the fires will cause turbidity. When large proportions of sulphates are present in the glaze, free silica will sometimes be formed and so cause turbidity; this may be prevented by the use of a slightly reducing atmosphere in the kilns up to just past dull red heat, and an oxidizing one for the remainder of the firing. In this way the sulphates will be decomposed before they have time to liberate the silica. Great care is required with reducing fires that no soot is formed on the surface of the goods, as this would become surrounded later by the melted glaze and no amount of heating would then cause its oxidation.

Twisted Ware. See 'Crooked Ware' and 'Warped Ware.' Under-firing is one of the great troubles of the clayworker. As a general rule, it is far better to over-fire than to underdo it, as in the former case probably only a few goods will be spoiled, whilst in the latter case none of the goods will be worth much. The whole question of firing requires careful study on the part of both masters and men. Some further details will be found on page 170.

Ünevenness in the glaze or engobe may be due to careless application, but it is not infrequently due to the running of the glaze. This latter is especially the case with ornamental ware in high relief. If the glaze is too soft or fusible it will tend to collect in masses in the hollower surfaces of the goods, whilst if it be too hard and infusible it will not flow evenly over the surface. Raw glazes containing soluble matters will usually produce an uneven surface, as these matters are carried to the surface in the drying of the glaze. The remedy is to frit such glazes.

Some glazes settle rapidly, and so an even mixture of their components cannot be applied to the goods. In such a case the addition of a little powdered alum is sometimes useful, or the addition of 2 or 3 per cent. of gelatin to the slip (the gelatin being previously dissolved in about four times its weight of nearly boiling water), will convert the slip into a stiff blancmange-like mass, which can be applied to the goods like a paint. Being solid, the material has the advantage that raw and unfritted colouring materials can be added to the glaze with the certainty that, so long as the gelatin does not melt, the colour must be applied evenly, no matter what is its relative density.

Unevenness in colour is chiefly due to 'settling' of the colour

ing matter in the glaze, to careless application, or to firing. In the first case, the use of some denser medium for carrying the colour is necessary (the use of gelatin, as just described, being suitable in stained glazes), whilst in the two latter more careful superintendence of the workers is necessary. If it is necessary

to have a number of goods of exactly the same shade, it is desirable not only to have them coloured at the same time and with one mixing of material, but to fire them in approximately the same positions and all in one kiln. If this is not done, the many factors which can influence the shade of ware will render it almost impossible to obtain a perfect match.

Vitrified Ware, in the sense of defective ware, is the result of fluxing material in or on the goods. It is sometimes produced when non-porous unglazed ware is made of clay which has been very badly mixed, so that the flux occurs in pieces instead of being properly distributed throughout the mass.

Patches of vitrified matter are generally the result of drops or splashes of glaze falling on to the goods. If the goods are fired at too high a temperature the material may partially melt or vitrify. Attempts have been made to utilise this effect in the production of self-glazed articles.

Warped Ware (see also 'Crooked Ware') is chiefly due to bad placing in the drying shops or kilns, to over-firing, whereby the materials are partially melted, or to faulty workmanship on the part of the maker.

White Patches or spots on coloured ware are usually due to accidental splashes of clay or engobe, or to the glaze being only partially fused. In the former case the defect is scarcely to be removed, though it may perhaps be painted over; in the latter, re-firing, possibly at a slightly higher temperature, will often effect a cure. See also 'Turbidity.'

Wrinkled Faces on bricks have sometimes been met with on the Continent in cases where the firing of the kiln has been erratic, and where a strong reducing atmosphere has made it possible for some silica to volatilise and to be deposited on the goods. This siliceous deposit may, apparently, be formed at a comparatively low temperature, and as it will not shrink with the contraction of the clay, wrinkles are formed. This is only a suggestion as to the cause the cure consists in keeping a watch on the air supply to the fuel.

CHAPTER XII.

WASTE.

THE subject of waste, though treated separately here, is only dealt with briefly, because many defects of working and management which produce waste have been pointed out in other sections, and the subject is too complicated to be compressed into a few words.

Waste may be considered in relation to goods, materials, power, and management, but, in whichever department of the works or offices it occurs, it is invariably included in the profit and loss account, though its identity is often hidden. In other words, the money value of waste is that by which it must be judged, for if this is not taken into account waste of one kind may be prevented, but a more serious loss of money may result from the 'cure' than was caused by the 'disease.'

Waste of Goods may be due to (1) over-production, so that the goods spoil, get damaged, or become out-of-date and, therefore, valueless; (2) carelessness in handling, whereby the goods become damaged or broken or dirty. Under this heading must be included faulty picking of the clay, or carelessness of the workmen in allowing food, paper, or other foreign matter to become mixed with the clay, and of the dippers, artists, etc. who get colour or glaze on the wrong parts; (3) so-called 'unforeseen' damage due to defective warehousing, draughty or leaky workshops, resulting in warped, cracked, or spoilt goods.

Most of the above defects and loss may be remedied by careful and thorough foremanship and management, ever remembering that 'a patch in time saves nine' when tiles come off the roof or a window-pane gets broken.

The question of dirt getting into the goods is a more serious difficulty in some works. It is so generally imagined that the cost of keeping the works clean is so much waste money, yet this is by no means always the case. In how many sanitary works, for instance, is the clay allowed to be trodden under the