Part V-Studies in Ancient Glasses and Glassmaking Processes

JSGT 1956 V40 T277-T300

The recipes recorded from 1700 B.C. to the seventeenth century A.D. for making non-lead-containing glasses all prescribe crushed silica rock or sand and ash (or glassmakers' salts) as the major, or as the only, constituents. Yet all analyses disclose the presence in the ancient glasses of from 2–3% up to more than 20% of lime; 0·2–0·5% up to 7% of magnesia and somewhat similar amounts of alumina and iron oxide; and phosphoric and other oxides (see Part IV). Analyses are now given of seven sands from possible ancient Egyptian glassmaking sites; and one Syrian, from the mouth of the River Belus. In only one (Assouan) is the lime less than 1% whilst in four it exceeds 10%. Alumina, magnesia, and iron oxide are also present in considerable amount, the lowest iron oxide being that in the samples of Belus sand (0·15%). Soda could have been derived from the natron lakes of Egypt or (less likely) from Nile water; soda and potash by burning coastal or desert plants, or inland plants and trees. The coastal or desert plants yield both sodium and potassium salts but with a much greater proportion of the sodium salts in a reactive form (e.g. carbonate). On the other hand, ash from inland plants contains much more of potassium than of sodium salts. Both types also contain lime and magnesia in amount greater, in some plants, such as beech wood, than the potassium and sodium salts; and also phosphoric oxide and silica. Accordingly, apart from solution of material from the melting pots, ancient glasses owe their complex compositions to iron oxide, alumina, lime and (frequently) magnesia derived from the sand, and soda, potash, lime, magnesia, and phosphoric oxide, as well as residues of sulphate and chloride, from the ash. The presence of substantial amounts of non-reactive sulphate and chloride in all the possible sources of soda or potash necessitated prolonged melting and heavy labour to make clear, homogeneous glass.

W. E. S. Turner