Friday, April 4, 2014

09 Plaster Production Molds Nº 16




Making a Plaster Build-up

I am using a Plaster Build-up instead of a clay build-up because the parting line along the bottom of the foot is very close to being a straight line. The plaster cast on top of the Formica table will be very smooth and flat. It will be used as the foundation for the other pieces of the mold.

This technique is described in the book Plaster Mold and Model Making (1973) by Chaney & Skee, from page 49 to page 61. I will be doing it in several parts, with this being Part One. I am using a carving wax foot as a pattern.

Click on any image to enlarge it.


I took the carving wax foot and set it in the middle of a piece of notebook paper. Then I used a pencil to trace around the outside of the foot. I tried to keep the pencil as close to 90 degrees as I could. This is called the Actual Profile. If I wanted to be very precise, I could have found the outside of the foot, using a square, by touching the square to the outside of the foot, all the way around the foot, and making a mark at each point until the outline was complete.

Next, I traced a 1/8th inch larger profile line around the Actual profile. I used scissors to cut the profile out around the larger outside profile.




I used a ruler to draw a line through the middle of the foot. I also drew a straight line on my plaster molding table. I aligned those lines, and traced around the paper profile of the foot. Then I measured from the widest and longest points of the Actual Profile, and marked lines 1.5 inches away. I used the ruler to make lines that formed a rectangle, 4.5 x 7.0 inches. This is the layout for the Plaster Build-up. This is one reason I like my Formica table. I can easily draw on it with a pencil.






I had wrapped a flexible tape measure around the carving wax foot, and saw that it was about 10 inches. So the next thing I did was roll out a an oil-clay ribbon that was 10.0 inches long by 1.0 inch wide and about 0.25 inches thick. I made the spacers from cereal box cardboard strips, stacked and taped together.






Next, I took the oil-clay ribbon and carefully put it inside the pencil profile of the foot. Then I put some reinforcing clay inside the ribbon. Once that was done, I started to put the coddles in place, using the 4.5 x 7 inch pencil rectangle lines as a guide.






I rolled out coils of oil-clay and pressed them along the bottom edges of the coddles as I placed them around the pencil lines. I used spring clamps to clamp the coddles together as I went around. I will be using the vibrator when I pour the plaster, so I don't want the coddles moving around when I do that.






The last thing I did today was apply  a Soap Parting Agent to the area outside the clay ribbon, inside the coddles.This soap Parting Agent is made from a 50/50 liquid-soap/water mixture. Because the coddles are sealed, and the table is Formica, I only had to apply one coat of Parting Agent. I removed excess soap with my brush. When this dries, it will be ready for pouring plaster.



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