POTTERY. 



137 



I'otiery. defended by saggars. This property of earthen 

 V^y-te.' it m.iy net In- impropt r to Mate, DAI I < n applied to the 

 construction of vessels for cooling wines niul other li- 

 quids, it having been ascertained Unit the water, by 

 pissing constantly from the inner to tin- outer surface, 

 is earned oil' by cvap >ration more hastily than could be 

 done on any other principle. 



"g. Heforc this pcriin -.-ibility, however, is removed, which, 

 as shall be shown, is accomplished by ;! /ing, the ves- 

 are to be printed, a process that mii-t be performed 

 while they arc in a state of hi.-cnit. T'ir designs are 

 engraven on copper-plates, and prints taken from these 

 as in common copper-plate printing. The surface of 

 the paper meant to receive th> -ion must be 



rubbed with soft soap. The colouring, whatever be 

 its hue, is, when diluted with some colourless earthy 

 matter, ground up with boiled linseed oil, until its con- 

 sistence when laid on the plate be that of soft paste. 

 The paper, covered, as just stated, with a thin coat of 

 soft soap, is now laid on the plate, and passed through 

 the rolling press. The printed parts of the paper, cut 

 out and moistened, are applied to the biscuit, and the 

 colouring matter is immediately absorbed by the poro- 

 sity of the biscuit. The paper being washed from the 

 biscuit, the colour will now be seen very distinct and 

 regular on the surface of the pottery. The colour is 

 generally made of oxyd of cobalt, which makes the fi- 

 gures of a bluish hue ; a colour which causes the 

 white of the vessel to seem more pure and beautiful. 

 This kind is denominated the blue and white ware, and 

 constitutes an important branch of the Staffordshire 

 manufacture. A small mixture of the oxyds of iron 

 and manganese imparts to the figures a dark colour; 

 which is sometimes done, and has by no means an in- 

 elegant effect. Printing, it may be observed, was for- 

 merly performed after glazing ; in which case they had 

 again to be subjected to fire, as in the porcelain manu- 

 facture. In some potteries, pencilling, that is, laying 

 on the colour with enamel after the glazing, was at one 

 time practised to a considerable extent ; but this mode 

 being very expensive, is now comparatively disused, 

 zing. The vessel being thus printed, its permeability must 

 be removed by glazing, or by covering its surface with 

 a vitreous substance. An almost endless variety of 

 materials may be used for this purpose, according to 

 the colour or the transparency required, or the quality 

 of the vessel that is to undergo the operation. One 

 species of stoneware is glazed simply by throwing sea- 

 salt (muriate of soda) into the furnace in which it is 

 biscuited. The salt, it is probable, is decomposed, the 

 acid flying off", while the soda combines with the earth of 

 the pottery, forming a vitreous coating. This pottery 

 might be extended for culinary purposes. From not 

 being in saggars, as previously stated, the vapour and 

 smoke come into immediate contact with the ware ; 

 and hence it is of a brownish colour. Pounded glass 

 also forms an excellent glazing ; it is very transparent, 

 and hence the colour of the vessel is easily seen through 

 it. Ware glazed with this latter substance is termed 

 cream-coloured, as it exhibits a yellowish hue from 

 the presence of a small quantity of oxyd of iron. 

 Glaze may be rendered of a bluish tint by the presence 

 of tin or arsenic, and a small portion of oxyd of cobalt. 

 Glazing, except in the first case, when sea-salt is used, 

 is uniformly performed by mixing the substances of 

 which it is composed with water, so as the whole may 

 assume the consistence of cream ; and the vessel, when 

 in the state of biscuit, being dipped into this liquid, 

 the water of the glaze is absorbed by the pores of the 



VOL. XVII. PART I. 





biscuit; and if the vessel lias been turned with sum- 1'otteiy. 

 cient regularity, a coat of glazing, of unhn.-i tint-knew, S ~Y~ " 

 will be dcpoiitid on i re. 'I he ve&tel also, 



when taken out of the liquid, mutt still be continued 

 to be turned, to prevent the gL/e !i ng into 



ridges. Tin \\..ie is again placed in the saggars 

 before, and removed to the kiln ; but the fire is neither 

 so strong nor so long continued a& be: on-, the object 

 being only to bring the glaze into perfect fusion. These 

 glazes, however, are subject to some objections, parti- 

 cularly their not expanding and contracting equally 

 with the ware; hence the vessels . known 



to crack, and even the glaze to peel off ; and the sur- 

 face thus rendered permeable to fluids as when in a 

 state of biscuit. The oxyd of lead, however, removes 

 this objection ; but this oxyd, even in its vitreous state, 

 and when combined with flint or clay, i easily soluble 

 in acids, and possesses poisonous qualities; go that it 

 is now as little used as possible, and bad consequences 

 have often taken place from eating pickles' kept in jars 

 glazed with lead. Lead, however, cannot be entirely 

 dispensed with. All the coarser kinds of pottery at 

 least are glazed with this substance, which, it may be 

 remarked, promotes the fusion and vitrification so ra- 

 pidly, that a very low degree of heat is required to 

 effect the operation. When the ware is removed from 

 the kiln, it is considered as finished, and as fit for use. 



The modern mode of glazing seems to be decidedly Roman 

 inferior to that practised by the Romans. Modern glaze, 

 glaze, as rlready shown, cracks, and often scales off, 

 and, besides, it is easily destroyed by acids. The Ro- 

 man glaze, on the contrary, from specimens of it seen 

 on urns dug up in several places, was entirely free from 

 this defect. The ingredients of which it was composed 

 cannot now be ascertained, though some have supposed tmktl 

 that it was made of some species of varnish ; while 

 others have insisted, on the authority of some vague 

 expression from Pliny, that it was obtained from bitu- 

 men. However this be, it is evident, at least, that it 

 never lost its original beauty, or, probably, that it im- 

 proves by time, and that it was so much valued among 

 the Romans that some statues were at length glazed 

 with it. 



There is, however, a species of very coarse porcelain 

 v;hich does not require to be glazed at all, and which 

 is of a yellowish tint, from a portion of oxyd of iron 

 being used in the composition of it. It does not un- 

 dergo glazing, because, without this operation, it is ex- 

 tremely and impermeably dense and compact in its 

 texture : a property which results from using a compa- 

 ratively small quantity of flint in the manufacture of it, 

 and from giving it a greater degree of heat than usual 

 in the burning. Glazing is really but a miserable imi- 

 tation of a polished surface; and the pottery in ques- 

 tion, scarce as it is, is the more beautiful on account of 

 its being devoid of the vitreous covering. This spe- 

 cies of ware is confined chiefly to bottles, particularly 

 those used for soda water and artificial mineral waters. 



A new species of pottery has of late been intro- K CW jpe. 

 duced, denominated lustre; which consists in fixing ciei of pot- 

 gold or platina on the surface of the glazing, in the tef y called 

 following manner : Dissolve platina in equal quantities iutl ' c ' 

 of the nitric and muriatic acids with heat. When to 

 this solution a solution of muriate of ammonia is added, 

 the yellow precipitate will fall to the bottom. Conti- 

 nue to add the latter till no more is precipitated ; drive 

 off the water by heat, and the powder thus obtained 

 must be ground with a small portion of enamel in oil 

 of turpentine ; and, after this preparation, it is in the 

 s 



