The secret of the salt-glaze process consists in firing the ware, specially composed of clay mixed with some siliceous sand or flint, to a temperature higher than ordinary earthenware will stand, and then, when red hot, shovelling common salt on to it through the top of the furnace. The salt fumes, passing through large holes in the saggars, cover the ware with a fine coat of colourless soda glaze. This glaze can always be distinguished from lead glazes by its peculiar pock-marked roughness, which indeed makes it somewhat unsuitable for plates or dishes for ordinary use; and, although for fifty years salt glaze did more than hold its own in public estimation, improvements in the old earthenware finally drove it out. By the end of the eighteenth century salt glazing had ceased to be practised.
Without Astbury,[30] who is said to have died in 1743, aged 65,[31] it is doubtful whether even salt glazing could have been a really great success. He it was that obtained a body white enough to show off the transparent salt glaze to the best advantage. Dr Thomas Wedgwood had only the drab body to work on—a far less effective medium.
With the object of whitening the clay body, Astbury began to import the white clays of Devonshire.[32] At first he used them only as a wash or dip to whiten the surface of the ware, just as the tin-enamel had been used to conceal and coat the coarse body of the Delft ware. Then he developed the use of the white sands of Baddeley Edge and Mow Cop to harden the body; and, in 1720, according to tradition, he made the really vital discovery of the value of calcined flint stones for both these purposes—to whiten and to harden the clay body from which the stone ware was made. Josiah Wedgwood, writing in 1777, attributed this discovery to a potter of Shelton called Heath instead of to Astbury,[33] but whoever it was that first noticed the whiteness of burnt flints, it was Astbury who first determined the value of the new material and the manner of using it. This discovery marks the first stage in the production of cream-coloured earthenware as well as in the production of the perfect salt glaze.[34]
Astbury and his son Thomas made red and black ware also, after the pattern of Elers, but with this difference, the ornamentation of Astbury’s red or black ware is generally done in white clay,[35] instead of in the same colour as the body; and this is one sign by which collectors distinguish these two makers. Robert (or John) Astbury was succeeded by his son Thomas, who had started potting at Lane Delf in 1725. Their name does not occur among the potters of the latter half of the eighteenth century, but Margaret, Thomas Astbury’s daughter, married Robert Garner, a master potter of Longton, who attained a considerable position.
Joshua Twyford (1640-1729), like Astbury, had his factory in Shelton; one stood on either side of the mound where the church now is. Twyford is best known for his stoneware, chiefly red and black in the style of Elers, but he is also supposed to have made salt-glazed ware.
A particularly full account of the potters of 1710-15, especially of those in Burslem, is preserved in a document drawn up by Josiah Wedgwood in 1765. He gives both the weekly cost-account of a typical pot factory of this period; and also a list of the potters’ names and the kind of ware they produced. The document is in his own handwriting, and it appears from a letter of Wedgwood’s to Lord Auckland in 1792 that he obtained the information given in this document by “having examined some of the oldest men in the pottery here, near thirty years ago, who knew personally the masters in the pottery, and very nearly the value of the goods they got up, fifty years before that.” ... “From these data,” he goes on to say, “I can pretty nearly ascertain the annual value of the goods made here at that time; which was something under £10,000 a year.”[36] He then proceeds to guess at the annual value of the trade in 1792, which he says may be between £200,000 and £300,000. I cannot help thinking that his estimate was purposely on the low side, for the manufacturers of this date always lived in fear of special taxation. In 1821 the export trade alone was worth £423,399 a year,[37] and in 1822 £489,732.
The document runs as follows:
“Men necessary to make an oven of Black and Mottled, per week, and other expences—
“N.B.—The wear and tear, master’s profits, and some other things are rated too high. £4 per oven-full is thought to be sufficient, or more than sufficient, for the black and mottled works of the largest kind, upon an average, as the above work was a large one for those times.”