The Glass Process

The Batch

The crystal-making process is an art that has been perfected over the centuries, and it starts with the batch. The batch is the term for all the ‘ingredients’ which, when mixed together, produce crystal. The main ingredients used to make the batch are silica (sand), lead and potassium (potash), a formula that has hardly changed for 300 years. The brilliance of crystal comes from lead oxide, which increases its refractive index but also softens the glass, making it easier to cut. Potash improves the strength of the glass and increases its clarity. To these raw materials, cullet – recycled glass – is added. Cullet is waste glass from factory processes and broken up items that have not met quality standards. Adding cullet to the mix uses resources efficiently by reducing waste, it also has a practical effect assisting in the melting process.

The mixture is carefully weighed and loaded into a clay pot in a furnace, where it is heated to 1400°C for 48 hours. Each pot holds 800kg and is made from specially formulated clay. Because of the extremely high temperatures needed to produce molten glass; the pots have to be highly heat-resistant. If they were to crack or crumble, they would contaminate the molten glass.

Moulding and Blowing

Teamwork and flexibility in the glasshouse is essential. Men work in teams of three to six and each member of the team has a specific task – such as blowing or foot making. While the techniques are similar, the skills required to produce the different pieces of art glass vary and it takes six to eight years to become a Master of this craft. The first member of the glasshouse is the blower. He inserts a blowing iron – a hollow steel tube about 4 feet (1.2m) long – into the mouth of the pot. The working temperature for glass is 1080 - 1150°C depending on the item being made. A smaller item requires a more fluid mixture so the temperature needs to be higher. By deftly rotating the blowing iron on the surface of the molten glass, and skilfully judging the correct temperature and right amount of glass for the article to be made, the blower takes a ‘gather’ on the end of the blowing iron. He withdraws it, blocks it into shape and begins to blow into it, forming a bubble. For larger pieces the blower then adds another gather to the first and keeps working the glass until he estimates the size is right. Then, still turning the iron, he lowers the item into a mould and blows into the glass. This is the final glass-blowing stage for a vase or bowl.

Stemmed glasses however need further work. A 'servitor', another member of the team, is given a small blob of molten glass by the ‘bit-gatherer’. The servitor starts to shape this into a stem, and then takes it to the base of the glass. Another gather is taken and added to the stem to form the foot. To keep the glass at a working temperature of 600-800°C, it is regularly reheated in a special furnace called a glory hole.

At various stages more molten glass is added to the point where the glass is held on the iron. This helps to keep it at the right temperature and stops it falling off. Pear wood is used to make all the tools, pallets, formers and moulds, because it is hard and has a smooth surface. Moulds and pre-shaped blocks are lined with cork, which is heat-resistant and also absorbs moist. When the blower is satisfied with the item, he cracks off the iron. The final working temperature of glass is approximately 500°C and it needs to cool to room temperature gradually, either in a kiln or a special oven known as a 'lehr'.

If glass is not properly cooled, strains can result, meaning a fault may occur. Items are then inspected and checked. If a piece is not up to exacting standards, it is rejected and broken up for use a cullet.

The next stage is crack off. Items are measured and scored with an industrial diamond. The glass is then heated at this mark, causing the ‘overblow’ (surplus) to crack off. The resulting sharp edge is ground down between carborundum pads, and passes through a flame, which melts the glass to produce a rounded rim.

Lamp Work

Lamp work is an ancient technique of using a flame to melt glass. It involves the process of melting glass in a hot flame on a steel mandrel. (The mandrel is first coated with a bead release agent so the finished bead will come off easily after annealing). The molten glass is wound around the mandrel until the desired size and style are achieved. The flame used to melt the glass is produced by mixing gas, either natural or propane, and oxygen. Thus the flame produced is hot enough to melt glass. A torch is the primary tool for the flame. This technique has been adapted to produce the sculptures.