The clay pots or crucibles used by glass-makers in Stourbridge were made from clay sourced locally. It is the reason that the Huguenot glass-makers settled in the area. The clay was particularly fine, without impurities that would otherwise cause the pots to fracture in the heat of the furnace. The shape of the pot suited the furnaces of the famous Glass Cones, and each pot held a secret……
After the Middle Ages the technology for making glass moved from simple ‘Forest glassmaking’ operating locally, to industrial production methods exemplified by the glass Cones, factories within a chimney. One of the drivers for this change was the progression from timber to coal as a source of fuel. Coal furnaces were easier to control and burned at much higher temperatures benefiting the quality and the quantity of glass produced.
The crucibles or melting pots used to melt the raw ingredients changed from smaller open pots to much larger vessels with closed in tops and a mouth or opening to allow the glassmaker to gather the glass. The clay needed to produce these pots also had to change.
With the far higher temperatures of a coal furnace ordinary clay with many impurities would not suffice. The glassmakers sought a fine quality clay without impurities, that would withstand the furnace temperatures without breaking. Impurities in the clay meant that the co-efficients of expansion and contraction within the material varied greatly causing the pot to fracture and the metal to be lost.
The geology of the Black Country yielded all the materials that the glassmakers needed, sand from local sandstone, limestone from Wrens Nest, coal in abundance, and a fine quality fire-clay, ideal for making the pots.
The ingredients to make glass needed to be clean and free from contaminants that would affect the colour and quality of the products. As Stourbridge moved to making fine quality lead crystal tableware, the source of the sand changed from local sandstone to fine white silica sand with few impurities; initially sourced from Scotland. The sand was washed, and calcined to burn off any organic matter, before it was put into the pots.
The furnace temperature needed to be high enough at around 1600 deg C to ensure the metal came to a rolling boil, expelling any oxygen, which would result in bubbles in the products, and to separate any remaining impurities rising to the surface of the metal, with the oxygen.
The pots were made locally and built up slowly allowing time to dry; before the top of the pot was closed in an annulus or ring of clay was put inside. As the material in the pots came to a rolling boil in the furnace, the ring of clay would rise to the surface of the metal and any impurities from the ingredients, that had also risen to the surface, would be pushed to the outside, leaving pure glass to be gathered from the centre of the annulus.
As the glassmakers worked their pot throughout their shifts, and the volume of metal in the pot was reduced, the impurities would become more concentrated in the remaining glass. This is when the quality was reduced and this remaining impure glass was known as tail-glass, or end-of-day glass.
It was this tail glass that glassmakers and their apprentices used to make friggers, that’ll be another Bit of Glass.