How Do You Make a Bottle From a Blob? Shaping Something That Is Never Still

Glass has no melting point in the ordinary sense, softening gradually over a range rather than changing state at a temperature, which is what makes it workable at all. Hot enough, it flows like syrup and cannot hold a shape. Cooler, it stiffens progressively until it will not move. Between those lies a working range where it can be blown, pulled and shaped, and a gather of glass on the end of a pipe passes down through that range continuously as it loses heat to the air. The maker therefore works against a clock that cannot be paused, returning the piece to a furnace repeatedly to restore the temperature, and every operation has to be completed before the glass stiffens.

The process follows a defined order and each step has a name:

• •Gathering, rotating a hollow pipe in molten glass to collect a mass on the end
• •Marvering, rolling the gather on a flat surface to shape and cool it evenly
• •Blowing, introducing air to create a bubble, with the wall thinning as it expands
• •Shaping with wet wooden blocks, paper, shears and tools while rotating constantly
• •Transferring to a solid rod so the open end can be worked
• •Annealing in a controlled oven for hours or days to release internal stress

The final step is the one that determines whether the object survives. Glass cooling unevenly develops internal stress, since the outside contracts while the inside is still hot and contracts later, and a piece left to cool in air frequently cracks or shatters, sometimes hours afterwards. Annealing holds the finished object at a temperature where the glass can relax internally, then cools it slowly enough through the critical range that no significant stress develops, which takes hours for a small piece and days or weeks for a thick one. Stress remaining in a finished piece is visible under polarised light as coloured patterns, which is the standard test. The same requirement applies to any glass object, and rapid cooling is used deliberately in toughened glass to produce controlled stress.

Blowing is one technique among several and each suits different objects. Casting pours molten glass into a mould, which suits thick solid forms and is how large telescope mirror blanks are made. Pressing forces glass into a mould with a plunger, which is fast, mechanisable and produced an enormous quantity of nineteenth century domestic glass. Drawing pulls glass into rod, tube and fibre, which is how laboratory tubing and optical fibre are made. Float glass, developed in the 1950s, forms flat sheet by floating molten glass on molten tin, which produces surfaces flat enough that no grinding is needed and which is how essentially all window glass is now made. Lampworking shapes rod and tube in a flame, which is the technique behind scientific glassware and small figures.

Blowing was invented around the first century before the common era, probably in the Levant, and it transformed what glass could be. Before it, glass was worked by casting, by winding threads around a core and by grinding, all of which are slow and produce small heavy objects, so glass was a luxury material. Blowing is fast and produces thin-walled hollow vessels from a small quantity of material, which collapsed the cost and made glass a household material across the Roman world within a century. The later consequences are larger still, since thin clear glass permits windows, lenses, laboratory apparatus and containers whose contents are visible, and the whole of optics and a great deal of chemistry depend on a material that could be shaped this way.

Glass softens over a range rather than melting at a point, and a gather passes through the workable range as it cools, so the maker works against a clock and reheats constantly. Blowing creates a hollow form from a small quantity of material, which is why it collapsed the cost of glass in the Roman world. Annealing slowly is what prevents a finished piece cracking from internal stress.