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Peter Angel 10-30-2013 06:14 PM

Pinholes in hollow wax
Hello all.

I am attempting to make a hollow wax to give to a bronze foundry for casting.

I am pouring hot brown microcrystaline wax into a silicone rubber mold, letting it cool until it forms a skin and then pouring the warm wax out again.

However, I am getting tiny pinholes in my wax.

Any suggestions on how to prevent this?


Andrew Werby 10-31-2013 03:54 PM

Re: Pinholes in hollow wax
Those are probably air bubbles. When you pour wax into a mold, there's a little air that will stick to the mold surface, particularly in depressions, as the wax solidifies. These look like little holes, and they're tedious to fill and smooth over, particularly if there are lots of them.

Try this: Instead of trying to do this in one shot with one pot, think of it as a two-step process. Bring one pot of wax to a higher temperature, like about 250 degrees F (be careful - this stuff will cause major burns if you get it on you - use gloves, goggles, an impervious apron and shoes). If it starts smoking, it's too hot, and can burst into flame. Keep the lid handy in case that happens.Use an electric deep-frying kettle with an infinite range control, so you can regulate the temperature better than just high, medium and low. Pour that into the mold (carefully, you don't want to introduce any more air than you can help). A metal transmission-fluid funnel is good for getting it to the bottom of the mold without splashing down the sides. Pour it out immediately. This will coat the inside of the mold with a very thin layer of wax which shouldn't have a lot of bubbles in it. (Occasionally, a new mold will out-gas a bit, so if it does have a lot of bubbles, then leave some of that hot wax in there a while to "cook" the mold, which should eliminate them next time.)

Let the mold cool a little while, like 5 minutes, then pour in some wax from another pot, which should be cool enough to just be starting to form a skin over the top (about 110F). You can either do this in another electric kettle, or just use a regular pot filled with wax you melted in your fryer that's had a chance to cool down some (if you find it's cooling prematurely, you can add wax from the hot pot to heat it up). This wax, while it's poor at capturing detail, is just right for building up a thickness in the mold. Pour it in and pour it out again. Leaving it in the mold doesn't really help; thin rubber protrusions (corresponding to crevices in the positive form) tend to collect heat and repel wax the longer it sits in there. If these areas are having problems collecting wax, a little air from a compressor can help cool them down. It will usually take about 3 applications of the cool wax to get a hollow casting of the right thickness for bronze casting. Let the wax cool down thoroughly before unmolding, and see if your results are any better. Check to make sure your thicknesses are correct by shining a light on the outside of the wax casting and looking inside - any very thin spots will be evident. Sometimes you can fix small ones with a dab of cool wax applied with a brush, but usually it's best to start over again and be more conscious of your technique next time.

Andrew Werby

Peter Angel 10-31-2013 10:13 PM

Re: Pinholes in hollow wax
Thanks Andrew. I'll give the two step process a try. Peter

Owen Weng 12-13-2013 01:14 AM

Re: Pinholes in hollow wax
Andrew gives very nice advise.

One more option to consider:
when I make wax like bust, we will brush wax onto rubber mold.
Try to use small brush, dip it into molten wax and brush on rubber slowly and gently, especially those "corners".
After brush one or two times, pour molten wax into the mold. Control the speed of pouring is important as it make get air into wax if you pour too fast.
Slowly rotate the mold and let the molten wax thicken. Pour out the wax.
Repeat the steps two or three times.
This can make nice wax.

This methods apply for sculpture not too small in size,like life size bust.

obseq 12-16-2013 12:26 AM

Re: Pinholes in hollow wax
Moisture in your molds can also cause pinholes to appear in your wax.

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