| Making a latex mold |
 I used latex for the mold. It was pointed out that it would have been much better to make a mold using RTV. I agree .. but I had the latex rubber already. The RTV mold would have been quite a bit more expensive I think. Hey .. what can I say. I’m cheap. In any case, I only needed two castings so it seemed to me that the latex mold was better from that point of view.
So. Five layers of latex and one layer of latex + cheesecloth. |
| Negative Casting |
 Since *unlike the RTV mold which is self-supporting” you have to support a latex mold. That means that I had to form a box around the master and latex mold while it was still on the tile. Plaster poured around it will support the mold when you are making a casting.
Everything worked great – until I tried to get the casting out. I couldn’t. The problem was that everything fit so tight it created a vacuum between the negative and the outside of the latex mold. I ended up having to break the negative. It wasn’t a major problem as I held it back together for the second casting like a giant jig-saw puzzle. The solution would probably be simply to cut the negative in half – or – probably a simply groove cut into the negative would have allowed air to get behind the latex mold. I remembered afterwards that people who cast plaster allow a passage to let them used compressed air between the casting and mold. |
 You can see in this photo how the casting was set up and how the negative supports the mold. You can also see where I had to break the negative.
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| A casting |
 Once I had a casting I could use it to see where I needed to remove scenery so everything would fit.
That’s it for the abutments other then putting some color on them and bridge shoes – but that’s covered in my thread on the On30 Plate Girder bridge .. what I made the abutments for. |
