If you are casting in gypsum either professionally or as a hobbyist you have probably been using plain plaster of Paris or more robust gypsum based cements such Hydrostone, Hydrocal or even the very strong Forton MG casting system.
All these materials have their advantages and specific usage. Like me, you have probably discovered that Forton MG is by far the more robust material and it has indeed become the standard in the body casting and life casting industry allowing artists to make solid and lightweight pieces.
However Forton is a proprietary casting system and as such is fairly expensive. Life casting artists have three main cost centres: alginate, casting material and advertising… Keeping your cost down without compromising with quality must be high on the priority list of all professional studios. I have therefore been experimenting with various alternatives to the expensive Forton system and have now settled to a casting mix that is much more affordable, almost as strong and in many respects, easier to use than Forton.
I have been using a mixture of PVA glue diluted in water as a sealant for some of my castings. It occurred to me that if I was using this mixture to soak plaster I could obtain a casting material with new properties, always curious to discover better, cheaper, faster ways to do things, I experimented with plaster of Paris and with Hydrocal at different mix ratio but have since settled with Hydrostone.
I mix PVA glue with water to a ratio of one to one and soak in that volume of liquid a maximum of two volumes of Hydrostone. I mix it gently by hand and then mechanically for 5 minutes as one would do for Forton or plain Hydrostone.
It is easy to mix and you will quickly obtain a smooth paste. I have never had the small lumps that are often appearing in a Forton mix. All in all it is easier and quicker to mix than Forton.
The pot life of that slurry is just about 5 minutes…this is more than enough to apply a first layer even in a large mold. Mixing it with very cold water will slow down your setting time. A second batch can be mixed and applied immediately. I paint it on the alginate or silicon mold. It is a real pleasure to work with. It is much heavier and stickier than anything else I have been using before.
My resulting castings are consistently of a very high quality with none of these infuriating air bubbles pin holes, fine cracks or, “runneth” that more liquid casting materials are often producing. One of the annoying aspect of Forton when casting it an alginate mold is that it is very runny for almost half an hour, as a result you often get a very thin first layer specially on the vertical part of the mold.. That first layer is very fragile and sometimes crack when applying the second layer, specially when the alginate skin does not adhere perfectly to the mother mold. No such problem with this formula. The mix is much creamier than Forton or plain Hydrostone and you will easily build a thick layer even against vertical walls or overhanging parts and undercuts. I have been able to cast an entire torso in 15 minutes where a similar mold would have taken me over an hour with Forton. Many times, when fiberglass reinforcing is not necessary, I have been able to cast in a single layer.
Demold time is around two hours. Like plaster it will warm as it cures and is fully set when it cools down again. Full strength is achieved after 24 hours and it is fully cured in a few days depending on ambient humidity.
I have not been testing comparative strength for lack of an appropriate test bench and methodology. It can obviously be reinforced with fibreglass to increase resistance to shock like you would normally do with a Forton casting.
Simple test show that it will become as scratch resistant as Forton once it has fully cured.
This is where my formula really shines and completely outperforms Forton MG.
22kg of Forton FGR powder (modified Hydrocal) and 10kg of VF812 resin will cost me $497 here in New Zealand. This is including the required amount of accelerator but excluding any other agent.
To make an equivalent batch of my formula I only need to buy 22Kg of standard Hydrocal and 5 litres of weather resistant PVA glue ( I am using Selley’s Aquadhere, in the USA a similar product is Weldbond.) There is no need for an accelerant and everything only costs me $146.
Weight for weight my formula is therefore more than 70% cheaper than Forton. Since you will probably use slightly more material than you would with Forton, it is probably only 60% cheaper. This is still a very significant saving. You can report these savings to your customers and become more competitive or chose to increase your margin, In both cases it will positively affects your bottom line without compromising the quality and robustness of your product.
The astute sculptor will immediately see some other usage for such a material. If you are building mother molds around silicon rubber or urethane mold, this recipe will allow you to build thinner, lighter and stronger mother molds much faster than by using standard plaster of Paris and much cheaper and much quicker than by using Forton.
It is probably not suitable for small size hollow casting as it is too sticky to be slushed out of a mold. It is also too thick to be poured inside a narrow mold. But it can be made runnier by reducing the mix ratio to 1 to 1.5 instead of 1 to 2.
Even though modern PVA glues are often advertised as waterproof or weather resistant, I would still use a properly modified Forton slurry for works designed for permanent outdoor display.
For all other applications, you will find it really easy to apply on the inside of an open mold, It does not run the side of the walls and can be pushed in small places like fingers or nipples.
Do try it… You will love it. I am no longer using Forton but for the largest job where weight is and issue.
Forton MG is a registered trademark of Forton BV, The Netherlands. Hydrocal, Hydrostone are registered trademark of USG corporation