My research into solid body casting has reached the final stage and I am now ready to start creating the final works in wax. Before I can begin the wax weaving I need to create my plaster cores from blocks of investment plaster, carve them into the required shape and fit a metal arm through the centre of each block.
The plaster suppliers and the foundry suggested a 38% plaster/water ratio but this recipe seemed to me to be too much plaster and not enough water. After careful working out it seems that this ratio of 100% powder to 38% water is the mix favoured by the foundry for their investments, so it must be correct. I still have my doubts as I generally use a 50% mix when casting my fine casting plaster blocks for carving.
The technical data sheet also suggested a water temperature of 20-30°c and a mixing time of 8-10 minutes and a setting time of 10-15 minutes.
I carefully measured out my water and made sure the temperature was between the desired temperatures, then began adding the plaster using the sifting method of gently scattering the plaster onto the surface of the water. When 2/3rd’s of the way through the measure of plaster, peaks began forming on the surface of the water, which generally means the water can’t take any more plaster, but I still had plaster left over. Deciding to stick to the provided recipe I continued sifting more plaster until I had added the required amount and it now sat upon the surface of the water and did not want to be absorbed.
Stirring the plaster to combine the water and plaster fully was very difficult and was the consistency of thick porridge, but after 2 - 3 minutes of stirring the plaster did become smoother and resemble a thick custard.
To remove the bubbles I placed the plaster mix on my new vibrating plate (or as readers of my blog will have read, my fat busting weight loss machine) for 5 minutes of vibrating to bring any bubbles to the surface.
After vibrating I left the plaster to set for 2 hours as suggested, thats when the problems occurred.
Unbeknown to me, as the plaster was setting it began to expand slightly, this would be fine if it had expanded vertically into the empty space in the bucket, but no, it expanded horizontally. The buckets I use for casting are rubbery so that it is easy to remove the plaster once set, but this new plaster pushed the sides of the bucket, which being rubbery stretched with the plaster. When I turned the bucket over after 2 hours of setting time and gave it a few knocks with the mallet and was surprised that it didn’t fall out as the fine casting plaster blocks usually do. This is when I noticed the slightly bulging sides and realised the plaster was not going to simply fall out of the bucket but was well and truly stuck!
After chiseling away at the sides for what seemed like ages, did the plaster finally give way and come out of the bucket. The bucket being rubbery simply went back to its original shape, but now I have a problem. I have a collection of different sized rubber buckets for casting and this new plaster is going to expand and thus get stuck.
I need to figure out a method of using my rubber buckets but prevent them from stretching.