Casting How-to
Oct. 8th, 2004 09:19 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
ltmurnauCasting with Room-Temperature Vulcanizing (RTV) Silicone
I start by making a plasticine positive. You have to use the plasticine that does not contain sulfur, you get it in art supply houses. The sulfur reacts with the silicone and it does not cure properly.
Next get some RTV silicone. It comes in tubes at the auto parts section of your hardware store: its legitimate use is to make replacement gaskets for engines. It comes in different varieties with different temperature tolerances. I use Ultra Copper or Ultra Red Hi-Temp, which can take intermittent temperatures of 600 to 700 F. Squeeze some RTV into a paper cup and mix in some paint thinner, when it is sort of a creamy-goopy consistency you can brush it onto the positive with an old paintbrush or lay it on with a stick like frosting a cake. It's better to paint on several thin layers than one or two thick ones: at this point you are trying to get the RTV into all the crevices and corners. Let the RTV cure for 24 hours or more, depending on the thickness of the layers you have laid on.
Time to cast. I put the mold in a dish of (ordinary sandbox) sand for support and as a heat sink, after treating the inside with powdered graphite (available at hardware stores for fixing locks or lubricating hinges) as a release agent. Talcum powder works too.
First a word or three about metal and alloys: two things are important, the alloy's melting point and the quality of the resulting casting.
Tin-lead alloy melting characteristics depend upon the ratio of tin to lead. The higher the tin content, the lower the melting temperature. Pure lead melts at 621°F and pure tin melts at 450°F. Solders that contain 19.5% to 97.5% tin remain a solid until they exceed 360°F. The eutectic composition for tin-lead solder is about 63% tin and 37% lead, and it melts at 361 F. (“Eutectic” means the point in an alloy system that all the parts melt at the same temperature.)
Solders with lower tin content are less expensive and primarily used for sheet metal products and other high-volume solder requirements. High tin solders are extensively used in electrical work. Solders with 60% tin or more are called fine solders and are used in instrument soldering where temperatures are critical. Obviously they cost more. I have used everything from cheap solder to old bullets and fishing weights, but mainly in the past I have used monotype metal which is about 50/50 lead and tin. A friend gave me about 20 pounds of it a long time ago and I am down to the last couple of pounds. The melting point of this metal is about 450 F, which is OK for the molds but lower is better.
I'm a real cheapskate, but I was getting leery of using all that lead (though it's obviously had its effect on my brain already). I tried the lead-free solder that's commonly available in hardware stores (it is about 95% tin) but it does not give satisfactory results. As noted, I recently got in an order of Cerrotru, an alloy of bismuth (58%) and tin (42%). It's more expensive than solder but has a melting point of 281 F. You can get other "cerro" alloys with melting points as low as 117 F but they have lead and in some cases cadmium in them, which spooks some people (though it's not as if you were going to go to sleep sucking on the pieces!). See http://www.alchemycastings.com/lead-products/fusible.htm.
Quality of castings: Pure tin gives a detailed casting but it is quite brittle. Alloys that are mostly lead don't take detail and look rather blobby. The Cerrotru I used last night made detailed castings: one property of bismuth is that it expands very slightly as it cools, so it tends to pick up detail.
Now, heat the metal. Depending on the alloy you are using, you may be able to get away with using just a hotplate. I've been using a propane stove or blowtorch. I heat the metal in a small ladle. Once it's melted and then heated a bit further (casting temperature is 25 to 50 F above melting temperature, not too hot) I pour it freehand into the mold and let it cool. Sometimes it's difficult to control the pour and get the metal into all parts of the mold so I use a small piece of wood to "brush" excess metal away or "tap" and push it into small areas. The excess metal just falls into the sand and the sand falls out when you re-melt the scraps.
I am starting to experiment with homemade pickling and patina formulas now. Last night I used a solution of 3 parts hydrogen peroxide (3% solution) and 1 part white vinegar (5% solution), and it reacted nicely with some of my lead-bearing castings to give a dark-grey grainy surface. However, this solution did not affect the lead-free castings at all.
Any questions?
I start by making a plasticine positive. You have to use the plasticine that does not contain sulfur, you get it in art supply houses. The sulfur reacts with the silicone and it does not cure properly.
Next get some RTV silicone. It comes in tubes at the auto parts section of your hardware store: its legitimate use is to make replacement gaskets for engines. It comes in different varieties with different temperature tolerances. I use Ultra Copper or Ultra Red Hi-Temp, which can take intermittent temperatures of 600 to 700 F. Squeeze some RTV into a paper cup and mix in some paint thinner, when it is sort of a creamy-goopy consistency you can brush it onto the positive with an old paintbrush or lay it on with a stick like frosting a cake. It's better to paint on several thin layers than one or two thick ones: at this point you are trying to get the RTV into all the crevices and corners. Let the RTV cure for 24 hours or more, depending on the thickness of the layers you have laid on.
Time to cast. I put the mold in a dish of (ordinary sandbox) sand for support and as a heat sink, after treating the inside with powdered graphite (available at hardware stores for fixing locks or lubricating hinges) as a release agent. Talcum powder works too.
First a word or three about metal and alloys: two things are important, the alloy's melting point and the quality of the resulting casting.
Tin-lead alloy melting characteristics depend upon the ratio of tin to lead. The higher the tin content, the lower the melting temperature. Pure lead melts at 621°F and pure tin melts at 450°F. Solders that contain 19.5% to 97.5% tin remain a solid until they exceed 360°F. The eutectic composition for tin-lead solder is about 63% tin and 37% lead, and it melts at 361 F. (“Eutectic” means the point in an alloy system that all the parts melt at the same temperature.)
Solders with lower tin content are less expensive and primarily used for sheet metal products and other high-volume solder requirements. High tin solders are extensively used in electrical work. Solders with 60% tin or more are called fine solders and are used in instrument soldering where temperatures are critical. Obviously they cost more. I have used everything from cheap solder to old bullets and fishing weights, but mainly in the past I have used monotype metal which is about 50/50 lead and tin. A friend gave me about 20 pounds of it a long time ago and I am down to the last couple of pounds. The melting point of this metal is about 450 F, which is OK for the molds but lower is better.
I'm a real cheapskate, but I was getting leery of using all that lead (though it's obviously had its effect on my brain already). I tried the lead-free solder that's commonly available in hardware stores (it is about 95% tin) but it does not give satisfactory results. As noted, I recently got in an order of Cerrotru, an alloy of bismuth (58%) and tin (42%). It's more expensive than solder but has a melting point of 281 F. You can get other "cerro" alloys with melting points as low as 117 F but they have lead and in some cases cadmium in them, which spooks some people (though it's not as if you were going to go to sleep sucking on the pieces!). See http://www.alchemycastings.com/lead-products/fusible.htm.
Quality of castings: Pure tin gives a detailed casting but it is quite brittle. Alloys that are mostly lead don't take detail and look rather blobby. The Cerrotru I used last night made detailed castings: one property of bismuth is that it expands very slightly as it cools, so it tends to pick up detail.
Now, heat the metal. Depending on the alloy you are using, you may be able to get away with using just a hotplate. I've been using a propane stove or blowtorch. I heat the metal in a small ladle. Once it's melted and then heated a bit further (casting temperature is 25 to 50 F above melting temperature, not too hot) I pour it freehand into the mold and let it cool. Sometimes it's difficult to control the pour and get the metal into all parts of the mold so I use a small piece of wood to "brush" excess metal away or "tap" and push it into small areas. The excess metal just falls into the sand and the sand falls out when you re-melt the scraps.
I am starting to experiment with homemade pickling and patina formulas now. Last night I used a solution of 3 parts hydrogen peroxide (3% solution) and 1 part white vinegar (5% solution), and it reacted nicely with some of my lead-bearing castings to give a dark-grey grainy surface. However, this solution did not affect the lead-free castings at all.
Any questions?
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no subject
Date: 2004-10-08 11:36 am (UTC)Instead, I just think it is really cool. Fascinating that an alloy of bismuth and tin melts at a lower temperature than either pure bismuth or pure tin.
no subject
Date: 2004-10-08 11:59 am (UTC)These guys live in Minneapolis and do sandcasting, mostly in aluminum.
I would like to learn how to weld too, but have no room or equipment.
no subject
Date: 2004-10-08 07:09 pm (UTC)no subject
Date: 2004-10-12 09:20 am (UTC)I also tried epoxy putty, it works but is lumpy.