iViscosity is temperature
Glass viscosity drops by orders of magnitude between 700 °C and 1150 °C. The simulation uses an exponential law, so 100 degrees is the difference between honey and stone. Every tool's effect is scaled by the local mobility of the glass.
iiThe piece is a stack of slices
The glass is modelled as about 190 rings along the pipe. Each one stores its glass cross-section, cavity radius, temperature, colour, stress and off-axis sag. Tools move glass volume between rings, so pinching a neck pushes the displaced glass out beside the jaws.
iiiWhy you keep turning
Sag is driven by gravity in the pipe's rotating frame. When the pipe turns, the pull keeps changing direction and cancels itself out into a small wobble. When the pipe stops, it adds up and the gather falls.
ivBlowing
Air enters through the pipe bore. Cavity growth is proportional to pressure and mobility, and inversely proportional to wall thickness, so hot thin spots run away from the rest. Glassblowers exploit this by chilling the parts they want to keep and heating the parts they want to move. A wall blown below about half a millimetre bursts.
vCooling and shock
Thin walls lose heat far faster than a solid gather. Stress builds from steep temperature gradients in glass below its strain range. A torch on cold glass, or a hot jack line next to a set wall, will crack the piece.
viLight
The rendering refracts the actual studio behind the glass, with slight dispersion between red, green and blue. Colour comes from absorption along the path through the glass (Beer–Lambert). Glow is an approximate blackbody from temperature, so the glass goes yellow, then orange, then cherry, then clear as it cools.