Plaster Layering

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The "layering" technique outlined here is a very useful way to achieve relatively "tight" control over the details of whatever one is modeling. Knowing the basic approach to laying out Celtic knotwork is also useful.

A selection of plaster models that have been used to make silicone rubber molds.

To make a celtic knot, start with a piece of paper with a "grid" pattern on it.

Overlay the grid with the paper on which the design is to be "laid out" and trace the pattern you have in mind. If you're not entirely sure of the pattern you want...just experiment. If you don't like something, erase it and start over.

Erase every position at which two lines cross.

Now, go over the entire pattern using an over & under approach. That should be your "mantra" as you follow each distinct line → over & under, over & under,... . If you wind up with an "odd pairing", you've made a mistake :( ... on large, complicated designs with multiple intersecting lines it can easily happen, so mind your p's and q's.

Lay the finished design on the plaster blank and trace it using a pencil with a relatively blunt tip. Applying a little pressure as you trace will produce indentation that is easy to see. Afterward you can trace the design directly on the plaster, which makes it even easier to see.

Using a sharp-pointed tool, score the plaster to either the side of the pattern trace. The exact width will depend on your design particulars.

If the plaster is dry, use a small brush and wet the working area. It seems to make the scoring process a little easier.

Using a tool with a small "chisel" tip, chip away at the plaster directly around the scored pattern. Again, wetting the surface seems to make the operation a little easier.

The piece after chipping is complete.....

I like to "bevel" each crossing of the knot. You can decide if this is something you want to replicate.

Now take a tool and scrap the surface of the pattern. By exposing "fresh" plaster, it helps to create a better bond during the layering process (the next step).

Put a small amount of water in a bowl and sprinkle some plaster into it. Don't overdo it in terms of plaster and don't mix it. It should be mostly water and a little bit of plaster.

Dip a small brush into the mix so that the bristles collect a small bit of plaster along with water. When you touch the bristles to the plaster model, it will "wick" the water from the brush and deposit a small amount of fresh plaster at the surface. THIS IS WHAT YOU'RE AFTER.

Continue dipping the brush and transferring plaster to the model. In very short order you will see the layering process build up.

Continue the process until you're satisfied with it. NOTE that there is a "time limit" on working with the watery plaster mix ... when it starts to act like its setting, just rinse out the bowl and repeat the process.

The finished design.

The design elements of this bird-feeder were made using the techniques described in this mini-tutorial. This particular concrete casting was made using white cement and a small amount of yellow iron-oxide pigment.

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