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What is spruing?

Spruing is the process of putting your wax pattern on to a wax wire, and attaching the wax wire to a sprue base of some kind. The purpose of this process is to hold the wax pattern up in the air so that when a flask is placed over it and investment is poured into the flask, the wax pattern is covered by the investment.

Wait, wait, wait a minute! What is a wax pattern? What is a sprue base? What is a flask? What is investment? What the heck am I talking about as if everyone knows what these terms and processes are. Sorry! lets go over the terms necessary for all to understand this "Class" on steam casting.

Lost Wax Casting: The process of using a wax pattern of something that you want to cast in metal by making a "ceramic" mold of it, putting this mold in an oven, and burning out the wax pattern. Therefore, the wax is lost! This leaves a cavity in the mold so that metal can be cast into it.

Wax Pattern: The object to be cast. A copy of anything made of wax or any other material that can burn, that is used to cast.

Sprue BaseSprue base: The bottom of a mold that holds the waxes so that they stand up in the flask (can) as the investment (mold material) is poured over them. In commercial casting this is usually a rubber sprue base. See image at right.

Sprue ButtonSprue button: The center part of a sprue base that forms a "funnel" for the metal to enter the mold. Wax patterns are attached to this button. See image to the left.

Sprue WiresSprue wires: When preparing the mold these are wax wires that hold the wax patterns up from the sprue bases. After burn out, they make the path in the mold for the metal to fill the cavity left by the wax pattern. See image at right.

FlaskFlask: A can or steel tube that is placed over the patterns before investment is poured into it to form the mold. See image at left.

Investment: A powered material that is mixed with water and then poured into the flask over the wax patterns to form a mold that will withstand the heat and casting procedures. I use Satin Cast 20, because it easy to find.

Crucible: Anything that is used to hold metal while it is being melted.

Getting the mold ready and spruing is very important in any kind of casting, but is the most important step in steam casting. If it is not done correctly, then you probably will not have much success. So, please follow these steps completely and very closely. Nothing is more frustrating than to take time to mold, burn out, and cast something only to get an unrecognizable piece of junk. If you have spent hours carving the item, it is even more devastating.

Wax PatternFirst, we need a wax pattern. It can be purchased from a wax pattern company through mail order.If you search for wax patterns, several dealers will come up. Or, if you prefer, you can make something. A quick easy method to make a freeform pendant is to carefully melt some wax. Any kind will do, even some crayons or old candle will do.

Wax Melted In A SpoonMelt it in an old spoon and pour it in to some cold water. It will make some interesting patterns. Vary the temperature of the water for different results, swirl the water before pouring in the wax for even different patterns.

Wax Master 1Wax Master 2After choosing a portion or carving off the unwanted parts, melt a hole with a heated wire (a paper clip will do) where you can insert a jump ring after casting for putting it on a chain. See images to right.

Spruing the wax pattern:

A You may want to make a "nudgit". This is a soldering tool/ wax tool that you might want to try. It is patent pending, or at least I am thinking about it, but go ahead a make as many as you like. Take a small wooden dowel about 1/4 inch in diameter, put a rubber thing on one end, put a "T" pin (called a "quilting pin" now and found in the craft section of Walmart) through this rubber end, cut off the sharp point and you have a great wax tool. If you like, you can sharpen it and use it as an pencil!

DiagramSprue the wax pattern by taking a half inch piece of the 8 gauge round wax wire and melting it on to the wax pattern. Attach it to the wax pattern in a place that is easy to clean. Some thought has to be given to positioning the wax pattern so that it will cast. To do this just think of the metal flowing though the wax wire and into the wax pattern in only one direction away from the sprue button.

Attaching SpruesTake your nudgit, heat it over the candle, and carefully melt one end of the round sprue wire. Immediately following this, attach it to the wax pattern.

To make sure that it does not come off when pouring the investment over it, I have a special way to attach it. After the wax pattern has been initially attached, heat your nudgit, and melt a little of the sprue wire and a little of the wax pattern on just one side of the sprue wire. Don't do this all the way around it, only on one side. If you go all the way around, the pattern will likely just drop off. Let it cool completely, and then do the other side of the sprue wire. Then let this cool completely. Note: you can purchase a sticky wax called treeing wax that can also be used for this.

Making the sprue button.

DiagramThere are two things that will make the difference between a good cast and no cast at all. The first is the size of the sprue button, and the second is the torch that you use to melt the metal. The sprue button is probably the most important, because it, also, becomes the crucible. If it is not large enough, it will not hold all the metal to be melted. If it is too deep, you cannot keep all the metal melted at the bottom of the sprue button, even though a larger torch can solve some of this problem.

Step One: Make a sprue base.

Cut off a large piece of aluminum foil and fold it several times to get it down to a 4" to 4" square and at least 4 layers thick. This is your sprue base.

Step Two: Make a sprue button.

Making the pouring cupTake a little clay and make it into a ball. Stick it on the middle to the sprue base and begin to smash and smooth it out, so that it makes a mound.

Step Three: Melt some wax on top.

Covering the top with waxCover the top of this clay mound with some wax melted from the sprue with a candle. We do this because it will make it easier to attach the sprue wires to the sprue button.

Here are the problems that makes this step so important. Remember some kind of force is needed to push the metal into the cavity of the mold.

  1. Problem #1: In this case it is steam and we have to control it.
  2. Problem #2: If the molten metal begins to flow down the sprue wires, before we apply the steam, the surface area of the sprue wires will cool it, and the metal will freeze in place in the sprue wire. Once this has happened it is, I think, impossible to get it melted again (unless it is pewter), because the entire flask would have to be heated to the melting point of the metal being used.

SAFETY HAZARD: If moisture is allowed to drip into molten metal, it will create an explosion and molten metal will be sprayed in every direction. The reason for this is quite simple: the water hits the molten metal and flows into and under the surface of the metal. It instantly turns to steam and expands. As it does this, it blows the molten metal in all directions with terrific force. Molten metal can be poured into a lot of water with out this happening, but if you pour molten metal into a small amount of water or on a moist surface, it will explode. I might as well tell you about this experience that I had that is related to this danger.

Warning: this is a true story, but is not completely necessary to read for steam casting.

As a college student, I had a job at night to pour lead pigs in a water cooled mold. These pigs were used for a Linotype Machine. The mold was safe because the water ran through it kind of like coolant in an engine. I was told never to have anything to drink in this casting room, so I did not. It was a room about the size of a small bedroom. (10x10).

One night I wanted to test this theory of explosion, so I took a coke with me to work with some ice in it. I had a plan! I poured four pigs of lead (about 100 pounds). I ran to the door, grabbed one small cube of ice, threw it on to the lead, slammed the door, and waited to hear an "explosion!" I heard a little hissing, but no explosion. I was disappointed, until I opened the door. Almost no lead remained in any of the four pigs--the mold was almost empty. BUT, every square inch of that room was covered with shiny splatters of lead. It was beautiful. Then it occurred to me that I was going to get fired, and it would probably even affect my grades if they found out. It was two in the morning and I started scraping! At six in the morning, I had it all cleaned up. If I had stayed in that room, I think it would have killed me. Instead, I got a 25¢ raise for being the first student to clean that room in 20 years! It's the story of my life. I am the luckiest, dumbest guy you'll ever meet!

Moral of the story: You cannot let water drip into the metal that you melt in the top of you flask that is now called the crucible. If it does, it is going to splatter everywhere, and you will probably not get a raise from your significant other for burning holes in you, your clothes, and any other keepsakes in the room. I will talk about this more when talking about the caster. By the way, I was casting pewter into an RTV rubber mold in my garage when I spilled some on the cement floor. It started running along the floor and began to bubble. Remembering my lead pouring incident, I backed away quickly. Sure enough, the moisture in the floor finally turned to steam and blew a golf ball size of cement and some of the pewter into the air!

We need to talk about this here, because we need to control the steam, and the size and shape of the sprue button (now the crucible) will do it for us. If the moist part of the caster touches the molten metal first, it will create steam mixed with the metal, and it will try to explode in all directions. We want the steam to force the metal in one direction: down and into the mold. This is relatively easy to do.

Solving problem #1.

Make the spure button deep enough to hold all the metal after it has been melted, so that the metal is below the top of the flask and the investment. This way, when the caster is pressed on to the flask, only the top of the investment hits the moist material in the caster. This does two things:

  1. It seals the chamber where the metal is melted so the steam cannot escape, and
  2. It creates steam in the air pocket above the molten metal.

This steam does not have anywhere to go but through the mold, and it pushes the molten metal down into the mold as it does, thus, casting the piece. Therefore, we need the sprue button deep enough to allow about a 1/4 of an inch above the metal after it is melted. However , if it is too deep, it will be nearly impossible to get all the metal melted because the investment will constantly cool it.

DiagramSo make the shape of the sprue button as this drawing.

Notice the 1/4 inch of investement around the flaskNotice that there is also a 1/4 inch of investment all the way around the edge of the flask.

This is the portion of the investment that touches the moist part of the caster and creates the steam. (not the metal!)

Solving problem #2.

Because the metal is melted on top of the flask with the sprue button acting as the crucible, we have to devise a way to keep the molten metal from entering the mold too soon. Then, as we melt the metal, it does not prematurely run down in to the sprue wire and "freeze". This happens because of the surface temperature of the investment cooling the metal. When this happens, the metal will block the sprues and prevent any further casting. To prevent this, we will use something called "surface tension." As with most of what I do and teach with metal, I try to use the laws of nature and physics to help me. Surface tension is a great one to use. An example of surface tension is a drop of water sitting on a pile of dirt as a perfect drop, without getting absorbed into the dirt. Then, you touch it, and it quickly disappears and mixes with the dirt. Or, I think, it is like the drop of oil on top of the water which will not mix. Then you add soap, it acts as a surface tension reliever, and the oil mixes with the water?

Anyway, if the metal is melted on top of large holes made by the sprue wires, it may flow into them too soon and freeze. If we make small holes, by using small sprue wires, the metal will sit on top of these holes and surface tension will keep the metal from flowing down in to the sprues too early. Then, when we apply the steam pressure it forces the metal down through the sprue wires and into the mold. Problem solved, I hope.

Attaching the wax pattern to the sprue base.

It is now time to attach the wax pattern to a sprue button, but before that, we need to prepare it to create the surface tension that we need.

Step #1

Split the wax sprueSplit the 8 gauge wire into two forks on opposite ends of the wax pattern. This is the quickest way to do this that I have found. You could twist four 14 or 18 gauge wires together to form a "quad pod," and then attach it to the end of the sprue wire if you like. It takes more time and patience. I have also been informed that using a small 1/4 inch piece of sheet wax for this purpose works too. I will investigate this at a later time and let you know. The information is in the October, 1998 issue of Rock and Gem. It sounded like a good article.

Step #2

Attaching the sprue to the baseAttaching the sprue to the baseAttach the split ends to the sprue base carefully. If you do not do this carefully, you will end up with large holes leading to the sprue wires. I found that by heating my nudgit and gently melting the wax on the sprue button, and then quickly putting the sprue wires into the melted wax works the best.

You can not go back and remelt this area. Doing this will only lead to making a mess and enlarging the sprue wire openings. Once this is successfully accomplished, you are finished spruing! We are now ready to "invest." Investing will be covered next. Put away your sprued wax pattern in a safe cool place until then.