Page 36 - Curriculum Visions Dynamic Book

(Below) A Roman beaker about 20 cm tall and one of the first examples of decolourized glass.
Getting furnaces hotter
By the 10th century pot furnaces had also developed further and consisted of three compartments: one to hold the molten glass and two others next to it to allow the glass to be cooled slowly once objects had been made. Finished articles were put in the hotter of the two finishing compartments for a while and then transferred to the cooler one before being taken out and left in cool air.
Interestingly, many of the pot furnaces were built without any way of getting a draught of air through them. That is why these furnaces never reached very high temperatures. Later on, glass furnaces would become very tall cones, some over 30 m high. The change in design was specifically to get a blast of air through the furnace and to increase its temperature.
Decolourizing glass
Although it was desirable to give glass attractive colours, it was also desirable to remove the natural greenish hue from plain glass that is caused by iron impurities. The Romans discovered that by adding antimony or manganese to the glass, they could get close to colourless glass. These oxides produce a complementary colour to the green of iron oxide, and that results in a neutral grey tinge to the glass. Otherwise, an opaque white glass (looking something like ground glass) was made using tin oxide.
In the 15th century Venetian glassmakers perfected the use of a manganese compound called pyrolucite in their glass. It became known as “glassmaker’s soap” because it “cleaned” the glass of accidental colours. However, glass with pyrolucite is not clear and colourless. It has a greyish tinge and is poorly transparent.
The Venetian glassmakers got over these problems by making articles in very thin glass in which the colour and lack of transparency would not be noticed. Again, the scientific limitations were overcome in
a way that resulted in glass of exquisite quality in craftsmanship.
36