Plaster Mixing Guide

Mixing plaster is not difficult. the biggest mistake is over addition of plaster leaving a rather gloopy mess to try and pour. Air wont release, cavities dont get filled. We supply British Gypsum Plaster, who give the following ratios by WEIGHT of PLASTER:WATER. Note for the higher grade plasters Herculite, less water is needed!

HERCULITE 2 : 100/42

So to interpret the above, 100g plaster to 33g water for Herculite Stone, but 70g water if using fine casting. That said its easy to mix plaster very close to these ratios by eye. It takes a little practice but if you know what to lok for its not hard. Follow these guidelines:

1. Calculate the amount of plaster you will need. Rough rule of thumb fill the mould with water and tip out into mixing bowl.
2. Sprinkle plaster onto the water, do not add water to plaster it will go very lumpy!
3. Allow plaster to absorb water and continue adding until the plaster is all but touching the water surface.
4. Mix thoroughly and try to to whisk or trap air.
5. Consistency should be close to cream
6. If to watery the plaster will release air very well but be soft and crumbly
7. If too thick then there will be air bubbles everywhere but it will be strong

For Fibrocem its basically the same proceedure though there are one or two extra points:

1. When initially opened the mix may contain lumps in many respects similar to cement. This does not mean the product is out of date or spoilt!
2. Ensure any lumps are broken up before adding to water to avoid any unmixed areas.
3. You may require a slightly thicker mix than plaster for strength, dont over add fibrocem or you will end up with many air bubbles at the face, its much about practice makes perfect.

For pictures and movies, hints and tips on mixing plaster visit or friends at