I've been trying to make GOOD blank eye castings for some time. Water-clear urethanes tended to be bubbly and yellow, styrene resins stayed tacky or put of fumes for weeks, and epoxy resins left surface residues that cracked applied paints.

I think I've finally hit on a good one though at last. It's a modified acrylic resin, very clear, very white, bubble-free, cures in 24hrs. Only small downside is it needs platinum silicone moulds as it fuses to tin-cure and isn't quite as hard as I'd like.

When I say it's not as hard as I'd like, I mean if I press my thumbnail into it hard, I can leave a line in it. It also picks up fingerprints in this state. Not great for long-term use.

The result is I've built a temperature-controlled oven for post-cure hardening, and it's starting to get promising results.

* The far-left is an uncured sample showing it's raw state & clarity. It has hardened up very slightly over the fortnight since it was first made.

* Next is a very yellow sample cured as specified by the data-sheet, a ludicrously high 120c for 2hrs.

* Next is 70c for 3hrs, giving much lower yellowing but still more than I'd like.

* Then is 60c for 3hrs, lighter still, still hardened perfectly. Surface slightly mottled texture.

* Next is also 60c/3hrs, but cured in-mould. Only rear open surface mottled. Hardened, very slight yellowing. Mould flex lead to striations on part of surface.

* Finally is a 45c/3hr sample. This had no noticeable effect and resin remained "soft".

These resins only partly cure. Only something like 80-90% of their chemical base turns to plastic when the catalyst is added. Post-curing applies heat to force most of the remaining mass to polymerise. Over-cooking it make it yellow though.

The mottling is probably due to it being such a small oven and being too close to the heat source (though the latest test used the lower heater & sheilded the mottled surface, so..).

Switching to a more flexible mould (13shore rather than 40) may give enough give in the mould to hold surface contact through the post-cure bake.

Further refining the temp/time options will also help narrow down a good sweet-spot that gives good clarity.

Working with such small samples, variations in amount of catalyst may also be skewing results. More moulds are required so control groups & accurate comparisons can be made.