Wax bondo and how to use it

If you ever hear an experienced glass caster say s/he’s never had holes in a casting…they’re either lying or have a very creative definition of “holes.” Such casting defects come to everyone who sticks glass in a mold and fires, which is why we have “wax bondo.” 

That’s what I call it, anyway. I think the technical term for it is “disclosure wax*” or MFA students studying glass casting may have a different term for it. It’s a sticky, purely organic compound that I use when “recasting,” i.e., correcting flaws in a glass casting. I also use it to “glue” one or more cast components together in a larger casting, as in SHOUT.

I’ve discussed it frequently in this blog but thought it might be useful to give it its own page. I originally learned about it from that master of glass castery, Hugh McKay. He got the idea by repairing flawed castings with bung wax, a sealing wax found in wineries, but quickly discovered he could save money making his own wax with a simple recipe:

• 1 part beeswax (purified, true beeswax)
• 1 part lard (Crisco will do**)

Place both in a metal bowl over a pan of water on the stove, being VERY careful not to get any in the water (or any water in the bowl). Heat the water until the compound completely melts together. Pour into a heatproof container and cool.

When cool, it will turn a creamy yellow and feel stiffish until warmed. It makes a great hand cream, by the way, leaving your hands incredibly silky.

Wax bondo’s usefulness in casting lies in the fact that it’s entirely organic, so can fill in for missing glass but burn out in the kiln. I can pack it into a hole in my casting, carve in whatever detail I like, and fake the shape I should have gotten the first time around in the kiln. The refractory plaster will cover the shape+hardened bondo to make a perfect mold, already filled with glass.

Once cured, I can fire the entire thing in the kiln. The bondo will vaporize around 700 degrees F (370C), leaving void(s) in the mold. This allows the softening glass to drop and fill in the bondo void(s), eliminating the defect in the original piece.

Bondo has a very low melting point, so it goes from liquid to rock-hard quite easily. There are multiple ways to apply it: First, I can warm it with the heat of my hands to pack it into holes in a casting, then stick it in the freezer to make it rock-hard for polishing.

Or I can melt the bondo by holding it over a flame or a low-temp hot plate, just until it’s flowing. (You don’t want it too hot; not only can it spatter dangrously, it can also catch fire).

Then pour the molten bondo onto your piece, in the area with the holes, and let cool. Once it’s starting to solidify, just take a smoothing tool or your finger, and blend it into the surface.

Finally, I can simply scoop out some room-temperature bondo with my finger, smear it into voids and pack it in so it sits a bit proud of the surface, and then go over it with a soldering iron or hot wax tool. I apply just enough heat to let the bondo start flowing over the surface of the sculpture, blending with my fingers until the piece becomes uniform.

With all these methods, you’ll want to do ALL your coldworking first. The glass you’re recasting should be as perfect as possible, Then make your bondo repairs and re-mold.

The bondo builds up the imperfect areas to match the good parts of the glass, smoothing everything together seamlessly so that it once again becomes a perfect model of your sculpture.

You build refractory plaster over the whole thing; the plaster doesn’t care whether you’re plastering over glass or bondo, it simply forms a negative of the shape. The bondo will vaporize in the kiln, leaving you with an empty space exactly the shape that you need, ready to fill with glass.

When I divest the mold, I’m (hopefully) left with a perfect casting, smudged with what’s left of the bondo. Typically, after firing I’ll find small amounts of greasy, brownish-grey schmutz on top of the glass, next to the mold. It wipes off pretty easily, leaving no trace in the glass, as long as I don’t use too much.

If the bondo completely fills the void I have minimal issues with air bubbles in the defect area, which is nice–there seems to be a bit of a vacuum there that pulls the glass in and redistributes it. (It also helps if you build the mold so you’re firing with a gravity assist, i.e., the bondo is on the bottom of the mold)

That technique is very useful for joining mismatched casting components into a single whole. Either way, I can coat the entire assemblage, bondo and all, with refractory plaster, add a small reservoir for filler glass if necessary, and refire (recast) the piece.

It’s not a perfect solution, of course. Use too much bondo, and it WILL leave an ashy residue in the glass. It’s not noticeable in pate de verre or dark-colored billet castings (or not very), but you’ll definitely see it in mid- to light-transparent castings. So I use the barest amount I can get away with.

And if you’re working with exact color placements in your casting, recasting with bondo will cause the glass to shift, potentially blurring lines and messing up your patterns. You can try applying color directly to the defect areas, but it’s difficult to blend it in invisibly.

I’ve been able to correct underfired pate de verre sculptures like the one shown by adding powder or fine frit, and then spackling over with bondo. I use the largest particle size that will work for my application (the larger the particle, the smaller the surface area and the fewer the air gaps), and pack it into the voids as tightly as possible.

Then I melt a little bondo and pour it into the affected area (being careful not to disturb the loose frit), smoothing it over. I’ll chill the sculpture to really harden the bondo, then texture and polish it until it’s ready to invest. Then I build my recasting mold and fire it face-down (so that gravity will push the glass down onto the mold).

This method does NOT add enough glass to make up for the missing volume in the voids; remember that powder loses at least half its volume when it fires down, so there’s no way you can add enough frit to a void to completely fill it in when fired. If you did, you’d be making a shape roughly twice the volume you wanted in the final shape, covered with bondo, creating a distorted final form.

What you’re actually doing is coloring in from the front of the sculpture, then allowing the glass to fill in from the back, masking any color differences.

That’s the only way I’ve found to correct a pate de verre sculpture and keep colors in the right places; if you know a better way I would LOVE to hear about it. (please please post in the comments)

If you’re working with billet on a transparent casting, you can set up a reservoir over your re-mold in the usual fashion, add a bit more glass to fill in the voids, and gravity will do the work for you. The glass will push into the empty voids left by the burnt-out bondo, and the new glass will flow in from the reservoir.

The bondo method will not make up for large chunks of missing glass–a hole much bigger than a garbanzo bean is probably going to leave a little ash in the glass. For bigger voids in transparent glass, I’ll cut a piece of replacement glass, a bit smaller than the hole, and stick in the hole, covering the surface with bondo. Then I’ll texture the bondo to blend in with the rest of the piece.

The replacement glass doesn’t have to be a perfect match, but you do have to make sure you’re not trapping much (any) bondo between glass and sculpture. You’re also more likely to generate bubbles in the area (since you’ll be trapping air between the two pieces of glass) so you need to adjust mold position and schedule accordingly.

It’s not a perfect solution, in that it’s prone to leaving bubbles in the glass at the repair zone. But it’s better than making a new casting. One of these days, when I’m back to heavy-duty casting, that’s one of the processes I plan to refine. Till then, let me know if you come up with a better solution.

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* To the best of my knowledge, real disclosure wax, used to find and fill holes in wax models headed for lost wax casting, will NOT work in this application because it’s primarily made of microcrystalline wax, a petroleum derivative. The key here is using purely organic material which burns out completely; inorganics will leave nasty, silvery-grey residue reminiscent of mercury blobs in your glass.

** Someone on Facebook asked me what I meant by “lard,” since this can be a bit of a US-centric concept. By lard, I mean “semi-solid fat” or “partially hydrogenated shortening,” most often typified in the US by a product called Crisco but truly, you can use solid animal fat or butter or anything of that ilk. The key here is that it absolutely must be 100 percent organic. If you can use it to make traditional “short crust” pie crust, it will work in this recipe.