GLASS MAKING

The first stage in the production of glass is the burning of soda plants in clay-lined pits. The plants are added gradually and quenched until the pit is full. The resulting substance is powdered and sifted to produce 'glass salt'. This is then added to a mixture of white sand and black ash. This mixture is burnt in a furnace and turned repeatedly. Once cooled, it is pounded, sifted and finally placed into furnace pots and fired for two days.

This produces molten glass, usually of a slightly green or brown colour. The colour can be bleached out with 'glassmaker's soap' or other colouring agents can be added (see stained glass). The molten glass is ladled out into pots ready for shaping.

The simplest method of shaping glass is simply to pour it into a clay mould. Alternatively, a moulded blank is produced and then cold-cut into the required shape. The more common technique, however, is to blow the glass into a bubble or 'paraison' and shape that while still partially molten. Using this technique, glasses can be made with multiple layers, or broad flat dishes up to two feet across can be produced. One jug can be inserted inside another, and a variety of shapes can be produced by drawing out lobes from the paraison.

Glass can be decorated in a number of ways, such as applying trails and blobs, cutting, tooling, painting or gilding. The most sophisticated technique, however, is casing and layering. An outer layer of glass is gathered onto a paraison of different coloured glass so that the walls coalesce at every point without sinking into each other. Once the glass has cooled, the outer layer can be carved away in a decorative pattern to reveal the other colour beneath.

STAINED GLASS

In the middle ages, glass could be made in blue, red, green or even black colours. Sheet glass was difficult to make, and so was produced only in small pieces. As a result, windows were constructed by holding together the pieces with grooved lead in a stone framework supported by iron bars. Simple though this method is, it can be used to produce recognisable pictures.

In the fourteenth century, yellow and orange were added to glassmaker's palette, and by the early sixteenth century any colour imaginable could be produced. Much larger sheets of glass could be produced, and leads and mullions were no longer required to hold them in place. As a result windows up to seventy feet across could be made, with detailed and realistic pictures. Inscriptions could be added into the glass itself rather than having to be scratched in and painted with enamel. With realistic flesh tones, landscapes and stippled shadows, Rennaissance glass could be used to produce pictures almost as well as paint.

LENSES

Convex glass lenses were first produced in the early thirteenth century, but were at first only a curiosity for which nobody seems to have thought of a practical purpose - possibly because their quality was too low. Convex spectacles for long-sightedness were introduced in the mid fourteenth century, but the concave glasses needed to correct short-sightedness were not produced until the late fifteenth century. For the most part, early spectacles were not worn permanently, and consisted of two lenses in circular wooden frames held together by an inverted V-shape to go over the nose.

The film Robin Hood: Prince of Thieves not withstanding, telescopes were not known during this period, even by the Arabs!