Making Plaster Test Patches

Keep track of the ratios of your various mixes. Slap the plaster onto the wall or a sample board. After the test patches have dried, examine the samples for cracking and strength. We dig into them with a screwdriver and bang on them with a block of wood. The same principles that apply to exterior plasters are also relevant to interior plasters (Fig. 15.4).

15.4: Plaster test patches on the wall at Sand Island

Ranger Station.

If cracking is observed:

• Reduce the amount of water, OR

• Increase the amount of sand, OR

• Increase the amount of fiber, OR

If the plaster appears chalky or weak:

• increase the amount of flour paste, OR

• use better quality clay.

We have found that with certain soils, the combination of the variety of fibers (particularly cellulose fiber) along with the flour paste, produces an unusual marbling effect that is quite beautiful. We suspect it is due to a chemical reaction among all the ingredients and the different drying rates of the fibers in such a thick layer of plaster.

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