Pewter casting. Questions for the experienced

[Darth Mule]

So I've been dabbling in pewter casting. I have been having a little trouble getting the casts to come out smooth and clean. I've been using RTV silicone molds, and the mold is smooth, but the casts always come out with pock marks all over the surface. Any suggestions??? Next step is to try out a plaster mold.

I'm just doing gravity molds. No centrifuge. Not gonna ever be one. This is a home run project and I don't have plans to use a spinner. Please narrow your responses to the barebones system this is meant to be. And if you have a great pewter source/type, please direct with a link.

Thanks in Advance,
Chris

 

[juno]

Are you using a Lee pot to melt the pewter? Exactly what kind of silicone (brand, etc.) are you using?

 

[Megatron]

pre heat the molds, like a 60w light bulb or even in an oven.
Use talc in the molds, lightly dust not to much.
make sure you are not over heating the metal.

 

[shuktuki]

if you use plaster of paris
make sure the mould is completely dry
any moisture in the plaster will throw
molten pewter into the air
not a good idea
hope this helps

 

[Darth Mule]

Let's see. I've been using the smooth-on Oomoo silicone.

I'm not using a lee pot, just a pot I bought in a thrift store that I wouldn't have to worry about messing up. Melting the pewter on the stove. A friend told me that was the way to go. Didn't say I needed a special pot.

I did try preheating the molds tho, and that didn't really help. I may be overheating the metal or using the wrong kind of silicone alltogether. It's a one part mold, where the back is completely open. Would I get better results with a closed mold?

 

[TridCloudwalker]

There may also be impurities in your metal, especially if you're using a home made setup. There is a product you can buy that you put into the pot, which will float all the slag and impurities to the top to skim out. However, for the life of me I can't remember the name of the stuff, and my spin casting machine is 90 miles away right now.

Also, you're not going to be finding any good deals on pewter ingots right now. The price of tin is higher than it has ever been in the 40 years my father has been buying it. A fifty pound box is going to run you over $400 from a wholesaler.

Though, should anyone find a cheaper source, please let me know.

 

[Megatron]

Let's see. I've been using the smooth-on Oomoo silicone.

I'm not using a lee pot, just a pot I bought in a thrift store that I wouldn't have to worry about messing up. Melting the pewter on the stove. A friend told me that was the way to go. Didn't say I needed a special pot.

I did try preheating the molds tho, and that didn't really help. I may be overheating the metal or using the wrong kind of silicone alltogether. It's a one part mold, where the back is completely open. Would I get better results with a closed mold?

looks like Oomoo is a Tin based silicone,
if you use silicone you want Platinum based silicone. It has silica in it and is more heat resistant.
I did silicone (71-20 from poly-tek) molds and it was fine but I later went to investment ( a plaster and silica like mix) one time use molds.
Try a plaster mold since its a open back mold.
It should work, just make sure its dry.

 

[drusselmeyer]

Go back to basics too. You may be way too hot. Try adjusting you demp down in increments of 5 degs or so.

Metals and gasses can boil just like water does. Cooling the metal can eliminate this problem.

 

[Darth Mule]

I was also looking at some of the lee pots and noticed that they seem to melt the metal alot slower than my stove. I have a gas range that melts a pewter ingot in less than 5 minutes. Pretty fast. So I figure slowing that may help. Also, there is alot of slag that I've skimmed off, so impurities may be high. All things I have to work on.

It also turns out that the oomoo silicone is a tin cure. So I'll try the platinum. I have 3 different buckles I want to cast up as well as dagger runners, so plenty of opportunity to experiment.

 

[SFPROPS]

pre heat the molds, like a 60w light bulb or even in an oven.
Use talc in the molds, lightly dust not to much.
make sure you are not over heating the metal.

Those things right there usually always help.

You HAVE to talc the molds. I've never had a good pull with casting metal without talc on the molds.

Also, once you pour don't jar the molds in any way. The metal cures from the outside in. If you jar the mold, some of the hot inner metal moves to the outside, forming defects in the already cured surface.

 

[Entropy]

Preheating the molds, as suggested above, can make a great deal of difference. When I've done castings in metal or plaster molds, I usually just throw the first pull or two right back into the pot. Later pulls are cleaner because the first ones warm up the mold.

I'm afraid I don't have any experience with silicone molds (yet). I've also seen some folks get some pretty nice results casting in Bondo molds, but I haven't tried it myself yet either.

I've used both Lee pots and just an old cast iron pot on the stove. If you're going to be doing lots of small castings, the Lee pot will likely save you time and effort.

If you're hand casting, it can take some time to get a feel for the right temperature, etc. The nice thing about pewter casting, though, is that you can cast over and over and just remelt your bad results.

 

[SicariusSpei]

I'm curious because I am trying to cast replica coins (I'm doing Dresden Files tetradrachmas) and I also want to do a 30 piece set for my church to use at Easter. I intend to do a 2 piece reusable mould and was thinking silicone would be best. Any advice/warnings?

By the way my pewter is a silver alloyed one (Tin 97%; Bismuth 2.5%; Copper 0.25%; Silver 0.25%)

 

[PoopaPapaPalps]

It's an old thread but I do have something to add to your question.

I use Smooth-On's Mold Max 60 to cast pewter things. It's silicone and has a 100:30 ratio of A to B (a digital scale helps). Like with most other silicones, talc powder can be used to pull more detail if the piece you molded has a lot of detail, but unlike others, the silicone does push outward and expand a little when due to the heat the pewter is poured into it. So, if you have two-part box mold, it'd be good practice to be sure that the two pieces are held together pretty well or else the pewter may spill through the cracks.