Investing May
May 20, 2009 by cynthia
With May in her second day in the kiln (of a projected five-day firing), I’ve been busy documenting the steps I’ve taken so far to produce her. I’m slowly refining the techniques I use to produce deep bas-relief portraits in pate de verre (what I call Vignettes), and getting closer and closer to reproducibility with any of these portraits.
May is one of the larger portraits I’ve done so far, and she’s the subject of a number of experiments to solve nagging problems in background color, mold integrity and better rendering of facial features. I’ve got several experiments going on with her that somewhat change the construction of the mold and packing techniques, so I thought I’d detail them here.
(BTW, in case you haven’t figured this out, these posts serve as my lab notes. A blog is a great way to create a searchable, illustrated journal of studio processes, with the added bonus of making it much easier to share with experts who can help–or to show someone else how it’s done. If you’re into documenting your glasswork, I recommend doing it in a blog–and please send me the links!)
This is a series of posts that takes the sculpture from concept to finish:
- The inspiration for the sculpture “May”
- The constructing of May’s clay model
- The making of a silicone mold of May
- Planning May’s mold
- Making May’s mold (this post)
- Packing May’s mold
- Decanting May’s test run
- May the second (final version)
- Taking the girls for a stroll
And now…making the actual mold.
Mold box measures about 16x20 inches. The glass surrounding the center hole rests on the wax supports; May's head, of paper-thin wax, will be attached to the inside of the cylinder so that no petroleum-based wax actually touches the top of the glass.
Setting up the model
As I mentioned in the previous post, I couldn’t just set May on my mold platform, pour the investment over her, steam out and start packing–she needs more careful construction. I carefully trimmed May’s head from the wax background, then cut and edge-ground a 3mm sheet of Bullseye 1401 crystal clear glass. In this piece, the sheet glass IS the background, and it’s thickened up and colored by layers of frit behind the glass to maximize color saturation and minimize bubbles. So I cut a May-shaped hole in the center of the glass and mounted the head inside it.
BTW, since the background is going to be a fairly smooth black, it would be just as easy to use a sheet of black glass instead of clear (and I may do that in the final work). Right now, though, I also want to learn whether a clear cap would let me build sharp, saturated imagery in frit without looking too much like a clear cap…so I’m using clear.
Cutting the sheet was trickier than I’d thought. First of all, I’m making an irregular center cut, away from the edges. I tried using Patty Gray’s circle-cutting technique, but I can’t get it to work well for asymmetrical holes. My Taurus ring saw does have a blade that lets you cut an inside circle without touching the edges, but the saw’s throat is only a few inches deep, not enough to reach into the center of this big piece of glass.
I finally settled for cutting the sheet in two and using the Taurus on all but the deepest parts of the shape. Then I pushed the pieces back together, held them in place with victory brown wax and started constructing the sides of the mold box. The Taurus left a saw kerf where I split the glass; next time (duh) I’ll split the sheet with a glass cutter to avoid having to fill in the kerf.
Side view of May's head suspended inside the glass. The wax is all underneath the glass sheet (or inside the hole) where it can be easily removed.
Mounting the head in the glass presented another problem: I had to be careful not to let victory brown wax get between the sheet glass and the mold. I wouldn’t be able to remove it, and more than a tiny bit of petroleum wax in a mold leaves a nasty, metallic-black crud that must be ground out. (Ugh)
That meant I couldn’t simply “glue” the wax to both sides of the glass to keep May’s head suspended in the middle of the glass. Instead, May’s head had to stand on its own, and the connections/ transitions to the glass had to be made of something that could burn out completely in the kiln, leaving no residue.
Suspending May face-down on firebrick allowed me to work on the mold box.
So I constructed a series of wax strip walls that fit the outline of the wax head exactly, glued those together with more wax, and constructed a hollow wax “box” for the reservoir. I put these together, then joined wax and glass with an organic wax, the “bondo” I make using beeswax and lard (in the picture below, the bondo is the white stuff). That way only the bondo, which burns out completely, would rest between the glass and the investment. The victory brown would stay on the underside, behind the glass, where I could easily remove it.
I smoothed the transition from wax to glass using a mixture of 50/50 beeswax and lard, melted together into "bondo." Bondo keeps its shape during investment, giving me a perfect mold surface, but burns out completely in the kiln. This allows me to continue the wax onto the top of the glass smoothly without leaving an icky residue on the final work.
With the wax-and-glass box completed, I made a few final corrections to May’s face and neck, cleaned it up and polished the wax…and invested.
Making the mold
May’s face coat was a mix of regular plaster/silica investment plus a couple handsful of EPK and talc. The combination gives me a hard, detailed surface that holds up better under the abrasiveness of glass packing. She’s got another two layers of investment behind that, and the center layer contains fiberglass mesh strips to hold this relatively thin mold together. Both outer layers are mixed with “chop,” one-inch strands of fiberglass, to add strength.
The investment's a little thicker than I like (let it slake a little too long), but since this is the first coat and I'll smooth it into the detail with my fingers, I'm not too worried. Ideally, though, the face coat consistency would be a lot runnier, about like crepe batter, at this stage.
Steam-out, as predicted, was a breeze. Most of May’s wax pulled off the surface easily when softened; steaming was really only necessary at the undercuts.
Once softened, I removed as much wax manually as possible to avoid letting it melt too long in the steamer. It works, as long as the wax is already coming away on its own. If I have to pry or tug at all, I stop and let the steamer do it, rather than risk damaging the investment.
Got the rest of the wax out, cleaned the mold by dumping boiling water in and skimming off any leftover melted wax. Then I trued up the edges of the mold, checked it for potential defects. I made some minor corrections to the surface, mostly filling in small bubbles, and cleaned off stray bits of wax from the glass background surface.
Then I schpritzed clean water into the head area and started laying in my surface color powders. I’ll go over that in the next installment.
Related posts:
- Billet plus mold equals … Remember awhile back, when I was petting a huge shipment of glass?...
Again, I am just amazed by your process. It is so intricate. I’m sure you’ve been told this before, but I really do think you should write a book.
I’ve done some documentation on my blog for the step by step, but my projects are so simple in comparison. I’ll send you a link to one of them, just for grins.
Hi there from Australia. It is just so refreshing to see a lovely glassie actually sharing their techniques. It is always such a closely kept secret for so many artists.THANKYOU !!!!!!!!!!!!! – I was hoping that you could help me with the dreaded plaster stick. I usually use 50/50 dental plaster and silica mix for my cast glass. I have used both Gaffer and Blackwood crystal glass and very occasionally I am lucky but most of the time plaster stick raises its ugly head. I make shells and have worn my arm out hand polishing !!!!!! I see you use EPK ( ?) and Talc -is that for plaster stick ??
Any advice would be so appreciated – Kindest regards Laura Epton.
Hi, Laura, and welcome. I haven’t used the dental plaster (I think here we call it hydroperm but I could be mistaken); I understand it takes a much sharper detail than regular pottery plaster, which is what I use. One of these days I’ll just do a post on what I’ve found working with plaster mixes, exact weights and all but for now…I always build molds in layers (which also helps if you don’t want to break your back dragging and mixing giant buckets of plaster to cover a big model).
For my face coat, which is usually about an eighth of an inch thick, I start with a standard 50/50 pottery plaster/silica flour mix. Then I add three things: EPK, which hardens the mix and enables sharper detail, talc, which aids in release and aluminum hydrate, which is probably also acting as a release but seems to brighten the glass considerably.
I vary the proportions depending on the level of detail in the model, but it’s generally 1 (EPK): 1(aluminum hydrate): 1.5(talc). I just finished the BEcon conference on casting, and several large-format casters I spoke with suggested diatomaceous earth instead of talc…so I’ll try that at some point.
Anyway, I sift those things into the water first, let them fall, then start sifting in the plaster/silica mix. I slake for at least five minutes, then mix. If I’m really concerned about detail on a final piece, I’ll use a “squirrel cage” mixer on an electric drill, but most of the time I’m too lazy and just use my hands. If it’s well-slaked, it works pretty well.
The mix is fairly loose at that point, and I use a paper cup to dribble it over the model. Once it’s covered, I’ll take a big, soft haik brush and push the mix into undercuts and detail. I try not to let the brush, fingers or anything else push through the investment to the model surface–too much of that and I get scum/sticking in that area.
Then I just keep pushing the mix up the model until it solidifies.I don’t worry about making it pretty and smooth. I think the next layer sticks better if there are rough areas.
Second layer is almost always fiberglass dipped in straight plaster/silica and laid into areas that need reinforcing. At the same time, if there are what I call core areas (areas where the glass will need to contract sharply around the plaster), I’ll make “spongemix,” which is plaster/silica mixed with either vermiculite or chopped kitchen scraps. (Anything organic–carrot peelings, oatmeal, rancid nuts, etc–as long as it doesn’t expand when wet). That gets packed into core areas inside the face coat. The face coat is too thin to hold those areas, usually, and the organic stuff burns out, leaving a spongy interior that will crush together when pressed.
The third layer is plaster/silica mixed with reinforcement (either fiberglass “chop” or grog).
Then I steam out as little as possible–I learned from the Rowe twins to make hollow wax models to keep the mold in the steamer for very little time.
I also make sure NOT to contact the surface of the mold with my fingers unless I just have to, to correct a bubble. My technique, pate de verre, is very abrading to the surface of the mold because I’m packing grains of glass hard against the plaster, so I need all the help I can get.
It seems to work. When a mold comes out of the kiln I can slip a thin-bladed kitchen knife into one side and literally lift the mold off the glass almost in one piece (even soda-lime frit pieces, which are the worst for scumming/sticking). The investment will break off in the undercuts, but pretty much pops off with a couple of jabs or a hard spray of water.
I’ll get a slightly roughened surface with pate de verre, about like 400-grit sandpaper, but it takes about 15 minutes with 400- and 600-grit wet-dry to make it very smooth and velvety. If I use soda-lime billet, there’s almost no coldworking needed, and the best surface usually comes with Gaffer–literally like a baby’s bottom.
Someone on the warmglass board pointed out that the mix I’m using is very similar to a good kilnwash, so that’s probably why it doesn’t stick. (And now that I’ve said all this the next mold will stick like crazy, you know that, right?)
But try it on some test samples first, and vary the facecoat proportions. If your surface isn’t too finely detailed, you can also apply a very thin coat of kilnwash over the mold–make a solution of about 1 part kilnwash to 6 parts water, quickly fill the mold and just as quickly dump it out. You should barely be able to see the kilnwash at all. That also produces a nice surface, but it’s more trouble and if you’re not fast it can clog up your detail. If my model is very smooth and doesn’t have a lot of fine detail, I can sometimes brush this mix on for the same effect.
Hope all that helps.