Glass: sand, ash and a temperature nobody could measure

Glass is made by melting silica — usually sand — with a flux that lowers the melting point, and a stabiliser to stop the result dissolving in water.

Pure silica melts at a temperature that pre-industrial furnaces could not reach. Adding soda, obtained from plant ash or from mineral deposits, brings it down to something achievable. Lime is added for durability; without it the glass is unstable and degrades.

That formulation was arrived at empirically over a long period, and the ash used — from coastal plants in some traditions, from inland woodland in others — produces glass with different properties and different colours.

Before blowing

Glass working predates glass blowing by a long way, and the earlier techniques are laborious.

Core forming built a vessel by winding molten glass around a shaped core of clay and dung, which was scraped out after cooling. Slow, and it produced small luxury containers.

Casting into moulds, then grinding and polishing — essentially treating glass as a stone to be worked cold.

Mosaic and cane techniques, assembling patterns from slices of drawn rods.

All of these produced expensive objects for a small market.

What blowing changed

Glass blowing appears around the first century BCE in the eastern Mediterranean, and it transformed the economics completely.

A blown vessel takes minutes rather than days. Within a relatively short period, glass containers went from luxury to commonplace across the Roman world, and glass tableware and storage vessels became ordinary objects.

It is one of the clearer cases in the history of technology where a single technique shifted something from elite to everyday.

The furnace problem

Everything in glassmaking depends on sustained high temperature, and that meant enormous fuel consumption.

Glasshouses were sited where fuel was available, and in several regions they were periodically relocated as local woodland was exhausted. Forest glasshouses in parts of Europe operated as semi-mobile enterprises for exactly this reason.

The furnace also had to run continuously, since heating and cooling it wasted fuel and damaged the structure. A glasshouse was a permanent fire requiring constant attention, and the working rhythm of the craft was organised around it.

Annealing — cooling finished work slowly in a separate chamber — is essential. Glass cooled too fast contains internal stresses and will crack, sometimes days later. The annealing oven is as important as the furnace.

Venice and the guarding of technique

Venetian glassmaking became dominant in Europe from the medieval period, and the republic went to considerable lengths to protect it.

Production was concentrated on Murano, ostensibly for fire safety and effectively for control. Glassmakers held privileged status and were subject to restrictions on leaving, with penalties for those who took the craft elsewhere.

Techniques developed there — including a very clear colourless glass, and elaborate filigree work using canes of white glass embedded in clear — commanded high prices across Europe and were imitated widely once the workers eventually did leave, which they did.

The other traditions

Islamic glassmaking produced enamelled and gilded work of extraordinary quality, and glass from these centres was traded across an enormous area.

Chinese glass developed differently, partly because ceramics filled many of the functional roles glass took elsewhere, and glass was used more for decorative and imitative purposes.

Window glass has its own history: crown glass, spun into a disc and cut, and cylinder glass, blown into a cylinder that was cut and flattened. Both produce sheets with characteristic distortions, which is why old window glass looks the way it does.

Stained glass

Colour comes from metal compounds added to the melt — cobalt for blue, copper for green and red, manganese for purple, and gold for a particular deep red.

Medieval stained glass windows involve cutting coloured sheets to shape, painting details with a fusible pigment and firing them on, and assembling the pieces in lead cames supported by an iron framework.

The craft has survived largely through conservation work on historic buildings, which requires the original techniques and materials, and which has kept a small number of workshops in continuous operation.

Now

Industrial float glass, developed in the twentieth century, produces flat sheets of extraordinary uniformity by floating molten glass on molten tin. It made hand-blown window glass entirely obsolete.

Hand glassmaking survives as art glass, as scientific glassblowing — which remains a skilled trade because laboratory apparatus is frequently one-off — and as heritage production in places where the tradition has been maintained deliberately.

The furnace economics have not changed, which is why hand glass remains expensive: somebody has to keep a very hot fire burning all the time, and that has always been the underlying cost of the craft.