Note 251 — Chapter XV · (XXIX.)

This corresponds with modern practice. The following is from a paper by Mr James C. Powell, who, as practical worker in glass, has been engaged with Sir W. B. Richmond in the decoration in mosaic of the vaults of St Paul’s. ‘The glass which is rendered opaque by the addition of oxide of tin, is coloured as required by one of the metallic oxides; this is melted in crucibles placed in the furnace, and when sufficiently fused is ladled out in small quantities on to a metal table, and pressed into circular cakes about eight inches in diameter and from three-eighths to half an inch in thickness; these are then cooled gradually in a kiln, and when cold are ready for cracking up into tesserae, which can be further subdivided as the mosaicist requires. It is the fractured surface that is used in mosaic generally, as that has a pleasanter surface and a greater richness of colour; the thickness of the cake, therefore, regulates the limit of the size of the tesserae, and the fractured surface gives that roughness of texture which is so valuable from an artistic point of view.’ (Transactions, R.I.B.A., 1893–4, p. 249).