OPOpenPithMake curiosity visibleChemistry lab / Fossil moulding

About this lab

From one ammonite to five matching pieces: master, mould, plaster cast, epoxy cast and painted cast.

Moulding is a chain of materials chemistry. Silicone cross-links into an elastic mould. Plaster reacts with water and grows crystals. The two parts of epoxy join in a fixed ratio to form a network. Each step passes the shape on, and passes the error on too.

The master here is itself a teaching copy, not a natural fossil. Moulding a real fossil changes the specimen, and conservators decide whether it may be done. A good copy does not mean the original was kept safe.

Every reading and every curve comes from one model. Stoichiometry and makers’ data are calculated by definition. The cure curves, the temperature rise, the bubble counts and the effect of film thickness are teaching models that show trends only.

Five findings

Five findings

  • Silicone mould: weigh parts A and B of Mold Star 15 SLOW 1 : 1. This 10 cm square mould takes 192.3 g, 96.1 g of each. Working time is 50 min and cure takes 4 h. The low-relief master has no undercut and the mould comes off whole. Knobs give a 1 mm undercut that the silicone can stretch past. Spines give a 6 mm undercut that locks a one-piece mould, so it needs a two-part mould.
  • Plaster cast: 100 g of plaster binds 18.6 g of water and makes 118.6 g of gypsum. With 200 g of powder and 90 g of water (45 : 100), 37.2 g of water is bound and 52.8 g is excess. The dry cast is 237.2 g, 18.2 % lighter than the mix. With 80 g of water (40 : 100) the excess is 42.8 g and the cast is 15.3 % lighter. In the model the 45 : 100 mix sets at about 27 min and can be demoulded at about 51 min. Weighed once a day, its mass is stable only on day 7.
  • Epoxy cast: for EpoxAcast 690, A : B is 100 : 30. A 9.5 mm pour takes 80.4 g of A and 24.1 g of B. With only 20 g of B for every 100 g of A the cure limit is 67 %. With 40 g it is 75 %. Both stay sticky. The maker gives 9.5 mm as the thickest standard pour, 5 h working time, 24 h cure and 0.2 % shrinkage. A 20 mm pour is over the limit and should not be made.
  • Measuring: the master has diameter 80.00 mm, umbilicus 34.29 mm and whorl height 26.78 mm. The plaster cast reads 80.26, 34.40 and 26.86 mm, 0.31 % large on average. The epoxy cast reads 79.76, 34.18 and 26.70 mm, 0.31 % small on average. Both are inside the 1 % limit. A plaster cast poured all at once shows only 8 of ten details and is rejected.
  • Colour and evidence: three coats of 10 µm make 30 µm and keep all ten details. Coats of 25 µm leave 6, and coats of 60 µm leave 1. Copying specimen colour needs a reference photo, teaching zones need a key, and life colour needs research on this species. If the evidence does not fit, the label cannot say it.

Model and sources

Model and sources

  • Ammonite: the whorl centre line is a logarithmic spiral r = R·e^(bθ). The whorl expansion rate is W = e^(2πb) = 2, so the radius doubles every turn. The whorl half-width is 0.4 of the centre radius, with 30 ribs and 13 sutures per turn. The last 0.62 turn is the body chamber, which has no sutures. The master, the mould and every cast use the same height field.
  • Silicone: the ratio, working time, cure time, specific gravity 1.18 and shrinkage (under 0.1 %, taken at the upper limit here) come from the maker’s page for Mold Star 15 SLOW. The cure curve is a sketch fitted to those two times. The undercut is worked out from the section. The 3 mm limit for a one-piece mould and the bubble counts are teaching values.
  • Plaster: molar masses are 145.14 g/mol for CaSO₄·½H₂O and 172.16 g/mol for CaSO₄·2H₂O, with 27.02 g/mol of bound water. The 40–45 : 100 ratio, the 20–30 min Vicat set and the 0.42 % maximum expansion come from the USG Hydrocal White data sheet. The maker’s wet and dry densities give an 18.2 % loss, the same as the 45 : 100 calculation. Hydration follows an S-curve with 18 kJ/mol of heat. The set time, the cooling and the drying rate are teaching values.
  • Epoxy: the ratio, working time, cure time, maximum thickness, specific gravity 1.10, shrinkage and refractive index 1.565 come from the EpoxAcast 690 data sheet. The cure limit is set by the scarcer part. The rate rises with temperature by the Arrhenius law, and the cooling time grows with the square of the thickness. The activation energy, the heat rise and the “sticky below 90 %” rule are teaching values. The temperature curve shows the mechanism only.
  • Acceptance: the size chain uses the upper limits of the makers’ data: silicone −0.1 %, plaster +0.42 %, epoxy −0.2 %. The caliper reads to 0.01 mm. The limits of 1 % error and nine of ten details are the classroom standard set in the research report. The details lost when pouring all at once are a set scenario.
  • Colour: the ten details are given depths of 60–400 µm, and a detail counts as lost when the film is more than half its depth. Both are teaching values. The evidence needed for each purpose and the label wording come from the research report.

No physical experiment stands behind this lab, and no number on this page is a measured result. Industrial resins are mixed by trained adults at a ventilated station: uncured epoxy part B is corrosive and part A can cause allergy. If you change the brand or the model, look up every value again.

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