Presets
Playback
Flake and furnace
Thickness and water content of the raw flake, the furnace gas temperature and how well heat moves into the flake.
Popping rule
How much steam pressure the layers can hold before they separate.
Time
Scrub through the flake residence time or press Play.
Layers
Model checks
The science
Why vermiculite pops
Raw vermiculite is a stack of thin mineral sheets with water trapped between them. In a furnace the water turns to steam faster than it can leak out of the narrow gaps. When the steam pressure beats the grip between the sheets they spring apart like an accordion, and the flake grows many times thicker. This is exfoliation, and it is why expanded vermiculite is such a light insulating and packing material.
1. Heat flows in from both faces
A flake of thickness δ is heated on both faces by gas at temperature Tg. Inside it the temperature obeys the heat equation ∂T/∂t = α ∂2T/∂x2 with α = k / (ρ c). At each face the heat arriving by the gas film is -k ∂T/∂x = h (Tg - Ts). The ratio Bi = h (δ/2) / k says whether the flake heats evenly (Bi < 0.1) or the skin runs ahead of the core (Bi > 1). The page solves the half-thickness with a fully implicit finite-difference scheme (unconditionally stable) and a symmetry condition at the centre.
2. Steam pressure
The pressure of steam in contact with liquid water is the saturation pressure psat(T). The page uses the IAPWS-IF97 saturation equation (region 4), valid from 273 K to the critical point at 374 °C and 22.06 MPa (above that the page shows the critical pressure): 0.101 MPa at 100 °C, 1.55 MPa at 200 °C and 8.59 MPa at 300 °C. The pressure rises roughly exponentially with temperature, so a modest temperature rise near the pop temperature creates a huge pressure surge.
3. Popping rule and expansion
A layer pair separates once its local saturation pressure reaches the interlayer strength Ppop; the matching pop temperature solves psat(Tpop) = Ppop. The fraction of the thickness that has popped, times the water content, gives the expansion ratio E = 1 + Emax fpop (w / 0.10) with Emax = 14 (a teaching law; real expansion depends on mineral type and grade). Bulk density after expansion is the raw bulk density (taken as 1,000 kg/m3) divided by E.
Model checks on this page
(1) With the surface held at the furnace temperature (very high h), the numerical centre temperature is compared with the exact Fourier series for a plate. (2) Energy conservation: heat that has crossed the faces equals the rise in stored heat in the flake. (3) The saturation pressure at 300 K, 500 K and 600 K is compared with the official IAPWS-IF97 verification values (0.00353659, 2.63889776 and 12.3443146 MPa).
Honest simplifications
Constant properties (no temperature dependence of k and c), no latent heat or moisture transport inside the flake, no pressure build-up model for the vapour in the gaps (the saturation pressure is used as a proxy), and an empirical expansion law. Flake sizes, strengths and furnace conditions are illustrative.
What to try
Pick Thin flake, hot furnace and see the whole flake pop in about a second. Choose Thick flake: the skin pops while the core is still cold, and the temperature profile becomes steeply curved. Lower the furnace below the pop temperature and watch nothing happen. Reduce the water content and the expansion ratio falls even though the layers still separate. Raise the interlayer strength to make popping harder.