Glassblowing: Viscosity, Pressure & Spin

Furnace & Blowpipe

Glass temperature 1050 °C

Blow pressure 20%

Pipe rotation 0.8 rev/s

Workable (soft)

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Viscosity η

-- Pa·s

Volume

-- ×V₀

Gravity sag

Spin / gravity

How it works

Molten glass has no melting point — it softens continuously. Its viscosity follows the Vogel-Fulcher-Tammann (VFT) law used throughout glass science:

log10(η) = A + B / (T − T0)
η in Pa·s, T in kelvin — simplified soda-lime-style coefficients

Near 900 °C the glass is near its working point (η ≈ 10³–10⁴ Pa·s, stiff but shapeable); near 1300 °C it approaches its melting range (η drops toward tens of Pa·s, it flows like honey). The gob is modelled as a mass-spring shell: the computed η directly sets spring stiffness, damping, and a permanent "viscous flow" rate that relaxes each spring's rest length toward its current length — hot glass droops and stays drooped, exactly as it does on a real pipe, instead of springing back like rubber.

Three real forces act on the shell every frame:

Gravity — pulls the gob downward, causing it to elongate and sag off-centre.
Blow pressure — an outward force on every triangle of the shell proportional to its area and surface normal, inflating a bubble exactly like breath through a blowpipe.
Centrifugal force — spinning the pipe at angular speed ω pushes each point outward from the spin axis with F = mω²r. This is why glassblowers constantly rotate the pipe: it distributes the sag symmetrically instead of letting one side droop and drip.

Enclosed volume is computed exactly from the deforming mesh via the divergence theorem (V = ⅙Σ v₀·(v₁×v₂) over every triangle), so it rises for real as you blow.

Molten glass gob Blowpipe

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