Tag Archives: sculpture

Sprue system – part one

A sprue is the passage through which liquid material is introduced into a mold. A system of sprues is designed to deliver metal to all areas of the mold evenly and quickly. Bronze has shrinkage as it  cools; A sprue can continue to provide molten metal to the casting, provided it is large enough to retain its heat and stay liquid, as metal in the main casting cools and shrinks.

I prepared four waxes to sprue up. I may try different arrangements and discover which work best. This is especially crafty work. There are a variety of ways that this is done, superstitious and occult, and I wish there were more people to consult about it. Much of the shop talk in a foundry is on this subject.

The instinct is to start in attaching wax rods to the obvious points where the cleanup of the sprue marks on the final bronze will be easiest, and joining them up at some single location which will be the pour cup. But working this way can make a weak and fragile system. It has got to be strongly integrated with the sculpture, and really there is no way that an attachment point of wax merely melted together will hold the weight of the sprue system through all the handling that going to follow. No, not this:


What is needed is a self supporting structure which can bear all of it’s own weight, and further lend support to the sculpture. Something like your hand securely holding onto something delicate.


Your hand is the strong thing; the sculpture the delicate. Build something as if from the wrist, being the common point where the pour cup will be, to the hand, where the fingers are rooted from, to the fingers, which aren’t glued to the thing they hold, but hold/grasp onto it by virtue of their placement alone. Like this:


Here’s how. Warm up a pot of water to gently warm, not melt the rods you’re using. Pliable, but not soft. This is lukewarm or body temp water.


In this case I’m using the largest diameter rods I can, (to move metal into the mold fast and to be a strong structure), sometimes larger than the attachment point allows. Flattening the rod into a oval allows it to be fitted onto a smaller spot while keeping its same volume. Shape the rod to the piece, directly and without angles. The liquid metal wants a smooth path to follow, without creating turbulence.


When you have it right, dunk it in cold water to hold that shape.



Your shapes good? Outline them on paper so when you do the next one you’ll have a template to follow from. Save a lot of time later, especially years or next week later, when you can’t remember anything.



Assemble these together. Not by trying to melt then together with a soldering iron or whatever, but by taking some of the warmed-up wax you have and really shaping a physical joint that makes them one piece. This is sculpture in itself. Isn’t everything?




Cool this assembled sprue system in water. It should be strong enough to stand on it own, and then some. Wrist, hand, fingers.



This super-strong thing will then be integrated with the sculpture. A hint: those connection points will always be weak points of attachment. I enhance them with small pegs I make by cutting a small wax rod to size and inserting it into a melted hole I made with a soldering iron. Later I will melt another such hole into the piece and the rod (it just happens to be red wax) will support the connection much better.



It’s not all precision and detail here at DG International Studio. I can do sloppy work too, when I want. I need some wax rods to use for sprues in getting these waxes ready to pour bronze into, so I’m going to make a plaster mold to pour my own instead of finding someplace to buy them. I fit a cardboard box with some brackets to hold some rods in a variety of sizes. Finding the parting line on a 1” rod isn’t too hard, but on a 1/4”, more so.




It turned out to be difficult to extract the rods from the mold cleanly, the parting lines weren’t good enough, and I broke off a lot of the detail around and between each of the smaller rods especially, so the wax casts aren’t very neat.


I use a knife and trim off each one neatly into a cylinder. Takes a few minutes each, but in a weekend I made a mold which gets me several sets of rods in assorted sizes, and if I’d done it properly, with a rubber mold, I still be working on the mold today, instead of sprueing the wax models already.


But typical of me, I just can’t leave it alone. Someday, I have a plan to make a rubber inner mold using these two plaster halves.

Wax cast proof

Casting a wax proof from the mold that is free of defects begins the process of making a bronze casting. Ideally, the wax cast is not solid, it is wasteful, and I have had problems with the wax expanding the eventual ceramic shell mold in the burn-out  kiln, cracking it. (ceramic shell investment casting) What I want is like this, another piece I’ve been working recently.


This is Victory brown sculptor’s wax. Usually dark brown, but in other off-colors lately from my supplier, which is annoying. It comes in ten pound slabs,


which I melt down in this kettle, enough to fill the mold fully.


The idea is to fill the mold, allow it to cool briefly, and pour out the excess, leaving a hollow form with walls about 1/4 inch thick. The trick is to get the wax to the right temperature; too hot, and the walls won’t cool enough, be too thin, and the hot wax pouring out will re-melt carry away the wax around the pour hole; too cool, and the wax won’t take on the surface details of the mold correctly, or flow evenly throughout the mold. By experience, the correct temp is between 155 degrees and 160 degrees. Of course it takes more heat than that to melt the wax, so use a kitchen cooking thermometer to measure as it cool to the right temp. Essentially it is the temp where the wax begins to solidify from a liquid; it gets soupy, and begins to build up on the sides of the pot, instead of staying thin and watery.

Mount the mold in something securely. A lot of hot wax is dangerous.


Pour it in a steady stream. For this mold, I let it cool then for three minutes.


Invert the mold, and pour out the remaining liquid wax.


This is the little mold for the two head parts to the sculpture. These get re-attached to the main piece later.


Allow time to cool completely. Opening the mold too soon, while it is still warm, can deform or tear the wax apart if it hasn’t completely re-solidified. Here’s the fine result I was expecting


Wow, I’m pleased. That’s a very complicated form to get from a two piece mold. I put a lot of planning and work into this, and the proof is in the quality of the cast. This wax needs very little clean-up to be ready for the bronze casting process. Mostly just cutting the spouts from the head and feet, and smoothing the reattachment points lines on the head parts.




I won’t actually use this one, it was a test. I’ll pour several more, and pick the one or two that I like best to cast bronzes of.

Oh, there is still a lot of work to do. I’ve had some time off, but now I need to push on, and get this done.

There are other sculptures filling up my head that I want to get started.

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.


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.

Parting line

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!