Casting plaster for Lost Wax - Part 4

I have now created 3 wax pieces that are worked over a solid plaster core to varying degrees of success. The process of making the core is very lengthy; preparing the casting bucket, mixing the plaster, vibrating the plaster, fixing in the metal arm and letting it set. Once set there is a lot of carving and shaping work to get the desired shape with a lot of waste material. 

Is there a better method?

I’ve had a few ideas of how to either reduce time or materials or both.

1. Balloons - Filling a strong balloon with the plaster that would give a nice smooth shape without the need to carve. How would the balloon withstand the vibrating? Could the metal arm still be fixed into the plaster?

2. Cardboard - Make the shape in thick cardboard, reinforced with parcel tape. The plaster is poured in and the card can be cut away once set. There still would be some degree of carving to finish

3. Lego Bricks - One of my artist friends who makes Glass art uses Lego bricks to make her walls for plaster casting. Would this be able to withstand the vibrations?

4. Sand Casting - Now I know this would not work as the vibrations would dislodge the sand.

5. Plastic Bags - Much like the ballon idea could a bag be moulded in some way by knotting off areas of the bag to create the rough shape? This would need support during vibrating, perhaps with foam. Or would the bag simply sag into the foam. Perhaps the bag could be suspended?

6. Latex Moulds - This is a method that once a shape had been designed could be used repeatedly to create a shape that is smooth, requires no carving and most importantly reduces waste and therefore reduces cost of materials. Only snag, I don’t know anything about making latex moulds.

Conclusion-I need to try some of these and research the others.

Woven Experiment No.65 - Larger open freeform without a core


Woven Experiment No.65 - Larger open freeform without a core

The aim of this experiment was to see if a larger form could be woven without the use of a core and if it would hold itself without sagging. It began with lengths of 6mm wax held together with figure 8 weaving in 3mm wax. The spaces left between the guide lines were then filled with crochet work.

There were movement in the wax and it required a more delicate touch with the wax, but the result is much more fluid than working the wax over a core. It is more organic in appearance and the weaving is more open and light.

My only concerns if the weave is too open, will this cause a problem for the foundry? will it cast well?

There is a degree of movement even after the piece is complete, so to hold the shape I have inserted a plastic bag in the inside space and filled it with polystyrene chipping, and then fastened the bag. It seems to be holding well, the foundry may even be able to invest the outer areas of the piece first, then remove the bag and its contents before completing the investment stage.

Weaving on a Plaster Core, the results so far

So far the results of the three weaves on plaster have been varied and have created more problems than solving them. Heres some of the problems I have encountered:

1. The lack of the visible negative space has meant it is very difficult to view the effect of the sculpture.

2. The plaster core is heavy and is mounted on a cross plate. The piece can be spun around but only on one axis and I have gotten so used to be able to turn my pieces in any direction to allow the weave to flow across the surface.

3. The underside of the sculpture sags so to prevent this happening each piece has to be bound with strips of plastic to prevent movement. Alternate fixing is using lengths of waxed thread but I am concerned about over using the thread incase it prevents a clean burn out at the next stage of the lost wax casting.

4. Using a thick guide line works well at the beginning of the weaving process to mark out the shape you wish to create with your weave, but when you need to anchor the main weave to the guide it requires a degree of give which it doesn’t have. If the guide line was first applied with ample room for anchoring the later weaving then this would solve the problem, but it creates another as the guide line would simply fall off the plaster core.

Does this method of solid core casting need to have a core of plaster?

Can the core be added at a later stage and a more flexible light weight core be used?

Casting Plaster for Lost-wax - Part 1

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.