Mansel employed Italian workmen in the first instance, and it appears that prior to 1623 he had set up works in Milford Haven, at Newcastle-on-Tyne, in Scotland, and other places. The Newcastle furnaces were the most successful, the others being practically failures. Mr. Nesbitt thinks that the success of the Newcastle-on-Tyne works was due to the new system of flint-glass making, which must be credited as an English invention. Flint or crystal glass is made of a mixture of silicate of potash and lead. It was known but imperfectly made by the Romans in their clear glass variety, which contained a small portion of lead. In the Middle Ages the glass which contained lead was called “Jewish glass,” and was generally used for painting on, as it was more fusible than other varieties which did not contain lead. But all authorities agree that the English invented a new product in their flint glass, which was made after many experiments at Lambeth in 1673, as “clear, ponderous, and thick as crystal.” Mr. Nesbitt infers that it was the use of coal in the furnaces instead of wood that led to the development of the process. When using coal the melting-pots had to be covered in the furnace, which lessened the heating powers and thus made the fusing more difficult. To put more alkali in the mixture would have helped it to fuse at a much lower heat, but it would have injured the colour and quality of the glass, so lead was added in certain proportions, which gave the requisite clearness and strength. All kinds of glass vessels and plate glass for carriage windows were made at Lambeth, under the management or patronage of the Duke of Buckingham. [Illustration: Fig. 307.—Stained Glass; Fifteenth Century.] Though there are no records of glass making in Ireland of a very early date, the glass beads and glass bosses which decorate the objects of Irish art, such as the crosses, croziers, brooches, book-covers, and the celebrated Ardagh Chalice, prove that the art was known in Ireland at least in the ninth century, if not earlier. Mention has also been made in old writings of this period of glass vessels for use in Irish churches. [Illustration: Fig. 308.—Window Glass; English, Fifteenth Century.]