Silicone back

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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

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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

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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.

Parting line

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I added vents at the ends of the arms. They are cocktail straws, snipped and buried halfway in the clay bed.

At last I’ll finalize the parting line around the piece for the mold halves. Prepare a variety of clay snakes, collect your favorite tools, and find a comfortable place to work with good lighting:

Also, have a good cup of coffee, and listen to Radio La 2×4 Buenos Aries. (http://www.buenosaires.gob.ar/la2x4)

The techniques I think works best is, having built up in a general way close to the piece, to now lay a line of clay along the remaining gap of about the same size as the gap or slightly larger, and simply press it gently into place with small tools.

This neatly fills and seals the gap, without applying pressure onto the piece which will make the clay line difficult to remove cleanly and without blemish to the piece later. The gap must seal completely.

One side done, halfway around.

And finished, including some keys which can help to keep the two halves in alignment. I don’t take keys too seriously like some people. I think the complexity of the plaster mother mold’s shape, is enough to keep the silicone in place, but people expect to see these in a mold so do a little of it, for example. I think channel shaped keys like this do more to help prevent material you pour into the mold, like wax or plastic, from leaking out at the seams while it solidifies.

Next, I’m ready to pour the silicone to form the blanket mold of the first side you see here. Exciting!

Filling

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I’m filling in the five holes which are on the center axis of the figure. This is just parting line on the interior of the figure. Not much to look at really, but these details are for my own notes.

I’m depending on having made the silicone blanket thin enough to peel-back, pinch-away from, extract, the mass of these plugged holes from the narrow V-shaped crevice they are in. If not, I’m still confident that the silicone will peel off with only some amount to damage to the wax, which I will eventually pour into this mold, in which case , I’ll repair the wax by hand before making the bronze from it. No big.

Open

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I had to go snowboarding again, sun and snow perfect, clear-minded oblivion. By now the plaster shell is really dry. I stand it up and very carefully, open it by inserting some wooden wedges and tap gently with a hammer. As soon as the suction is broken it comes apart easily. Do not pry. Be patient. I open the mold so as to reveal the first side that I worked on, that hasn’t seen daylight since the end of December. Nice, isn’t it? Face it, there is satisfaction in doing your craft right.

Now I’m going to remove that pretty clay blanket and reveal the model under it. I’ll use a palette knife to cut and separate the blanket from the plaster,

carefully peal back the blanket,

more,

and more, until the model is revealed, but still resting in the bed of clay which is the second-side blanket. I had done some work earlier on to roughly define a parting line in clay from this first side, when it was still in the cradle, and you can see some of that in place still. The task now is to detail that parting line perfectly.

I save the clay blanket that was removed, because I can weigh it and know roughly the amount of silicone rubber I’ll need later to fill the void between where the blanket was removed and the plaster mother mold. We’re some way off from that still.

I cut back the plastic wrap with small scissors, which has been helping to protect the model from the clay blanket to the existing rough parting line,

and tuck in the loose edges under the rough parting line. This way the plastic is still protecting the second side, you don’t need to remove the model from the mold at all, to get the plastic off, and it’s usually hard to do that without harming the model anyway.

There we are, ready to begin work on the final parting line. I’ll point out that the outlines will be pretty simple, but the holes through the middle of the body must be handled somehow and even though the silicone rubber material is very forgiving about undercuts and such, I’ll need to proceed with deliberation and reason.

Shell fini

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The box removed, and the plaster shell mold smoothed.

I’ll just let it cure, dry, and harden up a little longer, so when I split the two half apart, very gently, I won’t crack either half. The clay underneath undulates a lot, so removing the shells evenly, in one piece, using small wooden wedges, will be a trick.

Meanwhile, on the side, to use up the free time, I cast the Beachstones molds I made last Fall in urethane resin. (The waxes I made earlier from these molds for bronze didn’t work out too well and I’m going to do them over, soon I hope.)

It still needs finishing, but I’m very please with the way it turned out – that’s saying it came out looking like I’d hoped – like beach-glass, you know, that sand-smoothed broken glass you find at the beach. Look forward to more.

Back plaster

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I boxed up the mold, preparing to pour the plaster for the back side. I just build this quickly using foam-core, hot glue, and tape, lots of tape. I seal the inner seam of the box with clay where it meets the front side plaster so there is no leaking out of the new plaster. I line the inner side of the box with packing tape to release the plaster. I added the sprue and vents for the silicone to eventually pour into, in the form of cardboard tubes and soda and cocktail straws, but that step is still a way off (the clay under the plaster will be removed and replaced with poured-in silicone, forming an inner blanket which is supported by the solid plaster outer mold). It’s a lot of details with not much to picture.

Mix the plaster, something I’m not showing you, pour it on and shape it up. You can see some markings above which I use to estimate the thickness I want from the plaster.

I’ll let this dry a few days, take away the box and smooth it out some more.

It is a relief to be at this point. I’m maybe halfway done, but some of the most critical planning and preparation steps are complete now, and if I’ve done this correctly, most of what follows will be clean straight-forward work without anxiety.

matthew DG

Whatever I'm doing & sculpture

Silicone back

Half reveal

Silicone front

Parting line

Filling

Open

Shell fini

Back plaster