Monthly Archives: April 2013

Full reveal

There is nothing left but to remove the model from inside the two halves of silicone mold. Starting at a corner I can peel apart the mold gently all around. Because of the release, the two parts will separate.




The deep crevices, and where there are holes through the model’s axis, are the trickiest, but because of the thinness of the mold layer, as I had planned and hoped, the mold bends away and removes easily enough, and cleanly, without tearing the model up any. That should mean my wax casts later will come out undamaged and with the minimum about of touchup needed.Those holes through the axis were critical in my planning of this piece from the beginning, in both the design of the model and the mold, and I am grateful that it all turned out so well.

Silicone back-



Silicone front-



I surprise myself sometimes. With care and patience, I see that many difficult things are possible. The model, after so many months, now free, and in pretty good shape. And the two mold halves, complete. Complicated. It’s kind of amazing that all of that is two parts. The silicone rubber mold material is amazing.



The little block mold of the two head-pieces turned out well. I’d had a brief scare where I doubted if I could definitely recall that I had applied the release or not, but I had, it all came apart just fine. I had poured a plaster top for the mold.


Remove the box


Split the mold


Remove the models. Done.


I’ll clean and wash the molds next. Then, I can’t think of anything to prevent me from casting a wax from this, and that will begin another sequence of complicated events leading up to the bronze pour itself. But before that, I am so overcome with a feeling of relief about this project coming to success that I may want to take a short break and do something nice. The weather’s getter good finally, and this Winter’s worth of effort is completed. It would be a nice thing if I took my wife on a date.


Silicone back

I’m doing the second side, the back side, of silicone mold. This repeats the steps done for the first, front, side. Of course, the mold is turned-over now. Apply a liberal amount of release to all the surfaces of the two halves, the one with the model sitting in the silicone of the front side, and the other, back side plaster. Where silicone will meet silicone there must be a barrier of release or else the two halves will bond permanently. It is surprisingly easy to neglect this for some reason, when one is pre-occupied with so many details to consider. I apply paste wax (Briwax) to the plaster, several coats, and a spray type release I’ve mentioned before, which is hydrocarbons in ether, or something. Works fantastic. I’ll say it again: APPLY A LIBERAL AMOUNT OF RELEASE TO ALL SURFACES. If you can’t remember clearly applying release, then do it again.

Assemble the plasters, strap it together and again, cover the feet/pour spouts area, sealing it tightly so there will be no leaks.


And that box mold I’m doing on the side? I added some pour spouts there too, and applied release. I’ll pour the second half right on top of the first, with whatever’s left over in the bucket from the big mold.

Here’s the mold, with the silicone poured in. Like before, there are vent holes at the high spots which let air escape the mold as the silicone fills it. The clay plugs the holes when the silicone oozes out. I have again mixed up 250% of the volume of the clay blanket removed in the last step. It follows the formula: The weight of clay times 1.06 equals the volume cubic inches of the clay. The volume cubic inches times .68 equals the weight of the Mold Max 30 silicone product that I’ll need. The rational about qualities to mix up are all in the earlier post “Silicone front”. On that first side, it turns out I had a good bit of silicone material left over, even after the box mold was poured. Based on weighing the leftover amount, just doubling the volume of the blanket would have been enough. And although I know this, I’m still going to use 250% volume, to be safe. It is a large batch, as much as I can get into the vacuum tank at one time.


Enough to fill up the box too.


After curing for 24 hours, I open the foot-cover to see. It’s all good, but do notice that 10 grams of material has leaked into the left spout. The seal wasn’t perfect.


Split the mold open carefully, slowly, gently.



The mold, by chance, opened with the first side facing out. This side hasn’t seen the light before, and it’s our first look at it, but it isn’t the side we just poured. It looks good without any air gaps at the high spots – all the venting worked properly.


I want to free the other side. I invert the whole thing and suspend it on some cups to let it, sort of, ease its own way out, for a couple of hours.


Then I gently start to pry it away from the plaster shell…



..until it is worked loose.


Interesting. See the air gap there on the belly of the figure? Although there had been oozing silicone out of that vent hole before it was plugged, there must have been some subsidence of the material while it was curing, leaving that void.


I think it may be the same amount of material which found its way leaking into the foot-funnel. Or perhaps, because I forgot to seal the edges around the plaster half with the blue tape, air got into the mold and allowed the subsidence to happen. Otherwise, the clay plug should have secured vacuum to hold the silicone up. Who knows, but in this case it is easy to fix. I’ll re-assemble the mold and pour some silicone directly down the vent hole and fill the void. Since I will not apply any release to this area, the new materials will bond tightly to what’s there.


After 24 hours I open the mold and see that the repair is good.


Half reveal

Excited, apprehensive, now we’ll see the results. Removing the clay cover to the feet shows a full flow of rubber to that point; a good sign, and no air bubble at the high spot. Examination at each of the vent holes shows pink rubber filling. I am cautiously optimistic that my planning to vent the mold for this step has been successful.


Carefully open the mold. Don’t force. It’s said that it’s simply air vacuum that holds the parts so tightly together, not stickiness, at least not if you used a release properly. Once it gives a little, it will eventually open cleanly. Tap the wedges gently, wait ten minutes, tap more, wait, repeat.


With luck, the mold halves will separate exposing the clay side to be worked on next, not the silicone side, because I don’t want to remove the silicone from the model at all. In this case the clay blanket tore apart somewhat, but OK.




Remove all the clay, and protective plastic wrap. Save it all to weigh later to calculate the volume for the next silicone pour. The clay at the parting line actually removes easily from the model if you didn’t use much force applying it in the first place. It’s more difficult to get clay to stick to itself than not. Small amounts of rubber can leak through the parting line in places if it’s not tight enough. Just pull it off with the clay.




All the clay removed. The model’s suffered only slight damage which I’ll touch up. That’s why I laid the piece on it’s back first. it is this first side “down” which get the most wear during the process. The ‘up” side stays a lot neater.






Looks ready to do the second side of silicone. It may be easy. What a lot of work to get to here. What a relief to think I’m getting near the finish.

Meanwhile, in parallel development, the box mold has received a plaster top, and being turned over, the cradle and clay parting line is removed in prep for the next silicone pour.





Imagine my relief to be at this point in this project, and all’s gone well so far. I expected that I could do this, with enough foresight and patience, but I’m still very surprised when it works out.

Silicone front

I’ve been holding back something; remember that I had to cut off two pieces which make the back of the head to get this mold to work? I need to make a quick box mold for these, because, if I’m smart, I will be mixing enough silicone material to fill my mold up with some leftover, and I’ll have this second mold to use it up in. So, the usual procedure. Make a cradle, model clay up to the parting line, and box it in.




Next, I’m ready to pour the silicone blanket for the front side of the big mold.

It is especially important to apply mold release to all the surface of the clay model, and paste wax to the plaster shell that contains it; otherwise, the silicone rubber will stick tenaciously to both and make it impossible to part the mold later, destroying the work entirely. I use a spray-on product, Mann Ease-Release 200, which is hydrocarbons in ether base, that I get from Smooth-On, same place I get the silicone rubber (Mold Max 30) from . The paste wax is Briwax, a furniture wax with a lot of solvent of some kind like Toluene that vapors off quickly. I get it at a hardware store. USE RELEASE, over use it, and use it AGAIN. And if you can’t remember for sure, use it again. That’s where this high-end release is good; it doesn’t build up and alter the model’s surface like many other common household substances which are often use for release. I won’t even name them.

After that, I assemble the mold, and seal off the bottom with a flat clay piece, which is the pouring funnel area for the finished mold, to keep the silicone in. Wrap the mold seam with tape, to seal, and bind it up with a rubber strap, a cut up bicycle inner tube. They work great.



Next, I mix the silicone. I never have pictures of this because I’m too busy working quickly before the material passes it’s pot life. Here’s the summary: Decide how much silicone you’ll need to fill the mold. This is hard. What I do, is weigh the clay blanket that I removed earlier. Based on a lot of experience, I make some calculations. The weight of clay times 1.06 equals the volume cubic inches of the clay. The volume cubic inches times .68 equals the weight of the Mold Max 30 silicone product that I’ll need. Think about this. The clay is heavier that the silicone rubber, but the volume is the same for both, so I’ll want this formula to figure how much less of the silicone by weight I’ll need to fill the same volume as the clay. I derived it from the product’s specification sheets. Believe it, I have a whole notebook of these numbers worked out for each mold I make. There is more: Realize also, the clay blanket doesn’t precisely define the volume of the mold. It is like a loose drape over the sculpture. By much trail and error, I have learn that the volume of clay is about one half the volume of the space the silicone needs to fill. So double the volume calculation and work from that. You want to have enough to pour the whole mold at once. You do not want to have to mix up extra while the first batch is setting-up. You can, but it is stupid. I have done it too often, trying to be frugal with the cost of material. Just mix up what you know will be more than you need, and have some other small molds going which you can pour the extra amount into at the moment. It is better to mix up too much and throw it away, than leave the material on the shelf getting old past its expiration date. For this mold, I am mixing double plus another one-half, or 250%, of the volume of the original clay blanket. This mold is intricate, so I’m also using a silicone thinner at 5% of the total in order to help it flow better into and throughout the mold.

You can find instructional videos on the web which explain the proper mixing and pouring of silicone rubber material.

It helps a lot to use a vacuum pump to depressurize the air bubbles out of the mix. You’ll get a smoother mold without the chance of small pin-bubbles marring the surface details.


The silicone is poured in from the highest point of the mold. Various other high spots in the mold are vented to allow air to escape, so that the rubber can fill the mold completely. As each vent shows rubber flowing out of it, it is plugged with a bit of clay, so that the rubber will continue to fill higher and higher. There are eight vents to see in this picture.


On the side, is the box mold for the head pieces.


And some leftover. That’s good.


It’s a lot. A lot of planning, a lot of work, a lot of time. Truth is, it stresses me out, doing this. Sometimes, I think I should find another way to live my life.