The masters of wargames miniatures are sculped from various forms of epoxy — green stuff, brown stuff, sculpty and so on. Here are a few handy facts about epoxy resins, silicones, and other two parts materials from Joel Haas in the Yahoo Sculpting Group:
The handiest bit of knowledge: as a rule of thumb, for every 18
degrees Fahrenheit 10 degrees Celsius you increase or decrease the
temperature of the material, you will halve or double the curing time.
That means, if an epoxy is rated to cure hard in two hours’ time at
20 C or 70 F, then it will cure in one hour at 30 C or 88F; and, in a
half hour at 40 C or 106 F; and in 15 minutes at 50 C or 124 F. The
opposite is true as well –if you’re working on a mini and somebody
phones you, slip the mini in the freezer at about 30 F or a little
under 0 C and the mini will stay workable when warmed for about 8 hours!
I just kept a small toaster oven on my workbench set at “warm” (about
175F) and would slide one mini in to “cook” for a quarter hour or less
while working on another one. They come out a bit rubbery when
heated, but quickly regain their rigidity as they cool. Don’t heat
green stuff beyond 325 F, it melts at about 375 F!
Why is “Green stuff” rubbery? I was told years ago it was because it
was originally designed as plumbers’ epoxy to patch copper pipes.
Metal, of course, expands and contracts, so a completely rigid epoxy
was not as good a sealant for water pipes as one that had a little bit
of give.
Two part chemical reactions catalytic reactions –are
“exothermic”they give off heat as they take place. As we have just
seen, heat makes the reaction set up even faster. That is why you
don’t see large solid balls of epoxy or polyesters, etc. cast. The
heat from the reaction setting up is trapped in the middle and makes
the whole mass set up at different rates and times, producing
“crazing” and weak spots. With small scale casting, this is seldom a
problem. If however, you wanted to get the epoxy to flow more into a
mould with a lot crevices, etc, I suggest you cool the mould a bit so
the epoxy will stay fluid longer and get to all the parts of the mould
(don’t try this with urethanes).
If you have ever used car body repair putty (a filled polyester sold
under the trade name BONDO in the USA) for making terrain, you have
probably found it sets up in less than three minutes on a hot summer
day and takes almost 15 minutes in the winter. RTV silicones,
urethanes, etc. are all subject to the same heat sensitivity.
A terrific standard and short work on epoxies is INDUSTRIAL EPOXY
PUTTYS by John A Wills. Though published in the mid 1980s, I had no
problem finding copies offered for sale on the internet by typing in
“INDUSTRIAL EPOXY PUTTYS” +Wills on Google.
In addition to Polymeric’s epoxies, I have used water cleaned, rigid
epoxies by Aves www.aves.com and Magic Sculpt www.magicsculpt.com
A good way to keep you dental tools for working minis clean�especially
when working with green stuff�is to wear an old watch band with a
Velcro patch on it. On the Velcro patch, just place a small wad of
cotton soaked with mineral oil or baby oil and just keep wiping your
tool over that. A lot more appealing than licking your tool all the
time or using the oil off the end of your nose! Also, alcohol is an
epoxy solvent, so try that if you can stand the smell.
Joel Haas, sculptor