OPOpenPithMake curiosity visibleChemistry lab / Crystal cluster model

About this lab

Casting a quartz cluster model: from two jugs of liquid to a piece fit for a display stand.

Big crystal clusters in museums are often models. A silicone mould is taken first, then clear epoxy resin is poured in. This lab splits the job into five lessons: weighing, curing, heat, bubbles, and one honest question at the end. It looks like rock crystal, but is it?

Mix ratio, density, viscosity, maximum depth and refractive index are the makers’ values, tied to three products: EpoxAcast 690, EpoxAcast 692 and SORTA-Clear 37. Cure speed, temperature curves and bubble counts come from teaching models. They show trends only and are not measurements.

Five findings

Five findings

  • Weighing and mix ratio: EpoxAcast 690 is mixed 100 : 30 by mass. For a 130 g batch, weigh 100.0 g of A and 30.0 g of B. With 692 (100 : 40), the same 130 g needs 92.9 g of A and 37.1 g of B. In the model, 20 % too little hardener leaves only 88.9 % of the groups paired, and 50 % too much leaves only 80.0 %. Extra hardener does not make the cast set faster or harder.
  • Liquid turns solid: in the model, 690 at 21 °C reaches the gel point at hour 6.9, when the degree of cure is 57.7 %. At the maker’s cure time of 24 hours it is 95.0 %. At 15 °C the gel point slips to hour 10.7 and cure at 24 hours is only 85.5 %. At 27 °C the gel point comes at hour 4.5. At 21 °C, 692 gels at hour 20.7.
  • Thickness and heat: at 21 °C, 690 poured at the maker’s maximum of 0.95 cm peaks at about 28 °C in the model. Poured 3.81 cm deep in one go, it shoots to about 157 °C at hour 1.2. In 4 layers each one still peaks at about 28 °C, but the whole job takes 96 hours. 692 poured 3.81 cm deep peaks at about 32 °C, and at 7.62 cm deep about 122 °C. These temperatures are illustrative.
  • Bubbles: in freshly mixed 690 a 1 mm bubble rises 2.1 mm each second, while a 0.1 mm bubble rises only 0.021 mm each second. At the bottom of a 6 cm deep cavity, bubbles smaller than 65 µm cannot get out before the gel point. Slow stirring brings in 28 bubbles and all of them leave. Fast stirring brings in 249 and 65 stay. The silicone has a viscosity of 35 Pa·s, and a 1 mm bubble in it rises only 0.017 mm each second.
  • A model is not quartz: for the same 60.7 cm³ crystal, quartz weighs 160.9 g, the epoxy model 66.8 g and the silicone model 65.6 g. Quartz is 2.41 times heavier. Minimum deviation of light: quartz 41.1°, epoxy 43.0°, silicone 29.7°. Scale the model up 3 times and its volume is 27 times larger, its surface 9 times larger, and the epoxy model goes from 66.8 g to 1803 g. Each unit of volume keeps only one third of its cooling surface.

Model and sources

Model and sources

  • Makers’ values: EpoxAcast 690 has A : B = 100 : 30, mixed density 1.10 g/cm³, mixed viscosity 280 cP, pot life 5 hours, maximum depth for a standard pour 0.95 cm with a 24 hour cure, Shore D 80, refractive index 1.565 and heat deflection temperature 46 °C. For 692 Deep Pour the values are 100 : 40, 1.08 g/cm³, 370 cP, 12 hours, 3.81 cm, 72 hours, Shore D 80, refractive index 1.5372 and 38 °C. SORTA-Clear 37 silicone has 1.08 g/cm³, 35000 cP, pot life 25 minutes, cure time 4 hours and Shore A 37. Conditions are about 21 to 23 °C.
  • Mix ratio: the maker’s ratio is taken to match the two kinds of group exactly one to one. If the hardener is x times the right amount, the share of groups that find a partner is 2·min(1, x) / (1 + x). The lines between hard, soft and sticky are illustrative.
  • Curing: a first-order reaction, dα/dt = k(1 − α). At 21 °C the degree of cure at the maker’s cure time is set to 95 %. The reaction doubles in speed for every 10 °C (activation energy 51.5 kJ/mol). The gel point uses the Flory–Stockmayer formula for a two-functional epoxy with a four-functional amine, α = 1/√3. Viscosity rises as η₀ / (1 − α/0.577)². The molecular network is drawn on a diamond lattice. It is a sketch, not the real formula of any product.
  • Heat: a lumped heat balance with cooling through the top surface only. Heat of reaction 300 J/g, specific heat 1.8 J/(g·K), surface heat transfer coefficient 21 W/(m²·K). These three values are chosen so that the maker’s maximum depth sits on the safe side. A model peak of 60 °C or more counts as a runaway, and smoke is drawn above 130 °C. The red glow is a heat tint, not real light.
  • Bubbles: Stokes’ law with viscosity rising as the resin cures, so a bubble can rise at most v₀·∫(η₀/η)dt. The number and size of the bubbles are illustrative: slow stirring gives 50 with a median diameter of 0.3 mm, fast stirring 260 with a median of 0.09 mm, and dumping adds 60 more. In a thin stream, bubbles larger than 0.4 mm burst. Bubbles are drawn larger than life.
  • Quartz: a = 4.9134 Å, c = 5.4052 Å, density 2.65 g/cm³, Mohs hardness 7, ordinary refractive index 1.544. The angle between a prism face and a rhombohedron face follows from tan ρ = (2/√3)(c/a) and is 141.8°. For silicone the refractive index is a typical value for silicone rubber, 1.41. Light that enters one prism face and leaves through the next but one sees a 60° prism, and the deviation shown is the minimum.

Not offered as actions: vacuuming 690 or 692 (the maker forbids it), popping bubbles with a torch, pouring the next layer on half-cured resin, and adding quartz powder to the resin. The lab gives no light transmission, cost, service life or success rate. Those need real measurements.

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