My Casting Process Up Close – No It Is Not Instant!

I have been chatting with several customers lately at events, and it has become painfully obvious that some of them think that the casting process is almost instant, and that the cast pieces arrive looking as lovely as a shiny new penny.

Not really.

I was trying to figure out where they may have gotten this impression from. Maybe they watched an edition of “How It’s Made” where a giant machine spits out polished pieces, or maybe they have seen pewter casting.

So I thought about the Pewter Casting for a while. I do a little pewter casting for some of my reproduction spindle whorls. The originals were lead, and I am just NOT going to go there, so I use lead free pewter. I also make a few Pilgrim’s badges in pewter and bronze.

Cast Pewter pieces come close to looking like that new penny that we talked about, at least a lot closer than bronze or silver do when they are cast. This picture gives a nice perspective on casting simple pewter pieces. The original soapstone mold that I created for the scallop shell pilgrim’s badge (Santiago de Compostela in Spain), is on the far left. An “almost” cast ( the loop is incomplete and the “wings” on the top of the shell are not squared enough) shows what a fresh casting might look like. Above this is a row of six small spindle whorls, one of which still has it’s sprue intact. The larger spindle whorls on the right side show what flashing looks like (that wobbly bit sticking out on the edge of the spindle whorl).

pewter casting

OK, so let’s look at the process objectively. The mold is clamped, and the molten pewter is poured into the mold and allowed to cool. After three or four minutes the cast piece can be taken out of the mold. It will still be hot, especially if it has some bulk, like the spindle whorls do. Depending on the temperature of the room and workbench, a five minute wait is probably a good idea. Then I use a pair of sprue cutters to cut off the sprues. You can see where the sprues have been cut off on both the large and small spindle whorls. I then remove any flashing around the edges of the piece, either with a file, or a wet sanding machine. During this process the remains of the sprue are also removed. I then check any interior “holes”, like the interior of the loop on the scallop and the center hole on the spindle whorls for flashing, and remove it. Now the piece is ready for a quick polish. Twenty minutes in a tumbler with steel shot and burnishing compound will remove any oxides and give the pewter a nice sheen.

polished pewter

Shiny! This is what the polished pewter looks like. It is really not a complicated process, but even something as simple as pewter casting is definitely not instant.

Next Time: What Happens When We Cast Bronze

Casting Metal: Soapstone Molds – Part 5 – My Piece is Not Casting Well – What do I do next?

Diagnosing why a pewter piece is not casting correctly, without looking at the mold, is not easy. But there are a series of things that I always go through when I am having trouble with a mold. It is a process of elimination. Eliminate all of the potential problems and the piece should cast properly.

First, let’s look at the metal. Every metal alloy has a specific temperature at which it pours properly. Are you getting your metal hot enough? Does it flow well? Are you transferring the metal quickly from the melting pot to the mold? Are you getting your metal too hot? Metal that is too hot will put off a lot of fumes, which means that you are actually cooking off some the metal. This is not good for you or the metal.

Next let’s look at the general form of the mold. Do the two sides of the mold fit together properly? Does the metal leak out? If the metal is leaking out somewhere, and you are clamping it properly, then the faces of the mold – the two surfaces that are supposed to fit together – are not surfaced properly. You will need to resurface the mold to make the surfaces fit properly.

Is the funnel and the sprue large enough to allow the pewter to flow properly into the mold cavity? If the two sides of the sprue are not symmetrical or the sprue channel is not smooth enough the roughness of the mold may cause turbulence in the metal flow that will prevent the mold from filling completely. The inside of the funnel and sprue should be as smooth as you can make them and uniform. Be sure that the sprue does not narrow towards the cavity of the piece and that the transition between the sprue and the mold cavity is smooth.

Is air being trapped in the mold and preventing the metal from getting into the details of the mold? Before you consider making any permanent modifications to the mold try dusting the inside of the mold with caster’s talc or graphite dust. Simply use a small brush and lightly brush the entire inside of the mold with the powder. Sometimes that is all it takes to make a mold cast properly.

If this does not work it may be necessary to carve very shallow lines, about the diameter of two hairs away from the edge of the mold cavity to allow the air to escape from the mold. in some cases a larger channel, called a riser may be necessary if large quantities of air are trapped.

This graphic shows a couple of examples of air being trapped because of the design of the piece. The use of either shallow cut lines, or a riser should fix the problem.

soapstonemoldproblemsIf you have run through all of these points, and the mold is warmed up, and the piece is still not casting, it may be time to enlarge the size of the sprue or the funnel area. Begin by enlarging the funnel area, and then if the problem is not fixed, enlarge the diameter of the sprue.

I hope that you find this information useful in helping you to improve your pewter casting. This series was not intended to be a comprehensive reference to pewter casting, but rather an aid to understanding the process.