Overview: Test plaster mold in room temperature (80F) and warmed (100F). Test carving wax at 145F and 160F. Make 5 casts per test combination. Casting will be done of a plaster tile made with a 5cm line. Plaster mold was created 2 parts water to 3 parts plaster of paris. After pouring wax on the mold wait a few seconds then place in water to help cool. This will help decrease the wait time for tiles to be cool enough to remove from plaster mold so it can be cast again. Temperature is measured using a Radio Shack infrared thermometer. Accuracy of temperature measured by the device is unknown and not of great importance. An electronic caliper is used to measure length round to the closest tenth of a mm. The caliper is zeroed out prior to use each batch of 5 tiles.
Plaster mold room temperature (80F).
|Carving Wax Temp||160F||145F|
* The plaster mold line was stuck in the carving wax cast, but enough reminded on the plaster mold to continue using for tests.
Plaster mold warmed (100F).
This part of the test that I planned to do was stopped due to issues. Since the water used to warm the plaster mold as well as cool down the wax on the plaster mold were the same, this created problems in this scenario. Out of 5 attempts to create tiles, only 2 were removed properly. It is unlikely I will be repeating this situation when I do cast carving wax in plaster molds so there was no point to continue this part of the test.
Conclusion: If you take into account the 10 tiles I measured, the shrinkage range is 1.8-2.8%. The average shrinkage is 2.36%. The mode shrinkage is 2.6%. The median shrinkage is 2.4%.
(1 - (measured line length from cast wax / actual line length from plaster mold))*100 = shrinkage %
I noticed when I had my carving wax at 145F the shrinkage percentage was smaller than when I had the carving wax at 160F. Coincidence or is there something more to it? The difference is 0.24%. Not much.
Although my carving wax recipe did not have the same shrinkage as kwm's observations, it is not far off. While working with carving wax you tend to remove wax here and there also adding to the shrinkage. But at least take into account 2.5-3% shrinkage should help ensure your wax carving size is closer to the final size.
Now the question is how shrinkage works when recasting and using moulage molds. That is for another day.