The glass was made according to the following standard procedure. Raw materials for the glass were mixed using a laboratory mixer. One hundred grams of the mixture was melted in ceramic crucible, in a Carbolite electrical laboratory furnace. The crucibles containing the raw material mixture were placed in the furnace while it was still cold, to avoid thermal shock and cracking of the ceramic crucible. The melting was carried out at 1000-1200° C. in air. The molten glass was quenched in water to obtain the glass frit. The frit was dried overnight in a heating chamber at 120° C., then wet milled in a planetary mill to provide particles having a D₉₀ particle size less than 2 μm (determined using a laser diffraction method using a Malvern Mastersizer 2000). Wet milling may be carried out in organic solvent or water. The glass powder was dried in tray dryer and sieved.

Inorganic blends A to L were prepared by mixing the oxides, carbonates and nitrates using a laboratory mixer to produce a mixed material, followed by wet milling of the mixed material in butyldiglycol to produce a co-milled material. The milling conditions are set out in Table 4 below. The resultant blended powders were then dried in a tray drier and sieved.

Further inorganic blends A1 and A2 were prepared, using the same recipe and raw materials of composition A, by individually milling the separate components and subsequently combining them to form the inorganic blend. A1 was prepared by wet-milling the individual oxides, carbonates and nitrates separately, followed by drying each milled component individually in a tray dryer. The resulting dry powders were then combined and homogenised using a laboratory mixer to produce inorganic blend A1.