Plaster of Paris, how to cast in a latex mold/mould
April 23, 2010 1:42 pm craft techniques
You can use plaster of Paris in many ways, one of which is to cast it in a mold (mould) (see how to make a latex mould/mold). I created a latex mold from a clay object that I’d made, but you can also buy ready-made rubber or latex molds.
After you’ve made or purchased your latex mold, you can use it to reproduce your chosen object as many times as you wish. Depending on the shape of your object, this is probably the easiest and most satisfying part of the process.
The final result was a painted, relief, plaster of Paris art card (see left). I swap art cards with other artists and it is a good way of collecting miniature works of original art.
For this project you will need:
- plastic tub filled with small beads
- plaster of Paris powder
- water
- PVA/white glue
- plastic stirring spoon
- acrylic paints
- brush
Where can I buy plaster of Paris?
Dick Blick Art Materials sell plaster of Paris in various sizes. (Click on image below for purchase details.)
If you order over a certain amount, shipping can be free within the US, conditions apply. Orders placed from outside the US will have a different shipping charge and may be subject to import duty. (Click on the shipping information link at the bottom of the page on Dick Blick Art Materials for details.)
Casting with Plaster of Paris, step-by-step
Caution when using plaster of Paris
Be careful how you handle plaster. Use it in well-ventilated area and cover any surfaces, as it can get very messy. Take care when the plaster is setting, it can produce a very high heat. It is not advisable to encase your hands or any other body part (!) within the plaster while it sets! You'd be surprised at how hot it gets and it can burn you.
Step One
First of all, make sure your mold is supported, otherwise it can distort the look of the finished item. I used a plastic tub and filled it with small beads, but you could also use sand. Press the mold down so that it lies evenly.
Step Two
You need to have a mixing bowl, spoon, plaster of Paris and water all ready to use. Mix the plaster with the water according to the instructions. I mixed it to the consistency of single cream. Tip: Pour water into the mold and then into a bowl. This will tell you how much plaster of Paris to mix for one object. For extra strength, add PVA/white glue if necessary.
Step Three
Pour the mixture into the mold, ensuring that there aren't any lumps in the mixture. The best way is to wet the mold first, then pour halfway. The bubbles should come to the surface if you tap the mold. Pour to the top. Tip: I also add a strip of gauze or bandage to strengthen the object, if it is quite thin.
Step Four
Once the plaster has completely set (usually 20 minutes depending on its size), remove the mold. This can be difficult around areas that are higher than others. Gently roll or peel off the mold. I discovered that part of the plaster had an air bubble and, as you can see from the finished picture, it left a hole at the top right corner.
Step Five
You'll be surprised at how much of an exact copy the finished article is, although when the plaster dries it shrinks a bit. Now you need to let it dry completely. This can take up to a week. Tip: If you can't wait a week, place it in the microwave on defrost at a very low temperature for 10-15 minutes, depending on its size. Use oven gloves to take it out of the microwave, or leave to cool before touching. It gets very hot.
Finished Fat Cat art card
This is the finished, painted, plaster of Paris art card. You can paint it using acrylic (water-based) paints. I decided to call it Fat Cat.
Verdict for casting plaster of Paris in a mould
Pros Plaster of Paris is incredibly easy to use, as long as you are aware of the hazards. It gives very satisfying results. If you cast it in a mold, the plaster will replicate fine details really well. It can be sanded when dry to smooth any areas.
Cons Plaster tends to form air bubbles and it is sometimes difficult to get it to flow into those awkward nooks and crannies of a complex mold.
Liz :
Date: April 25, 2010 @ 11:43 am
Hi An Archway article on the local arts is a great idea – go for it girl!!!
frank cohn :
Date: July 9, 2011 @ 7:47 pm
hi, where can i get information on how to create copper relief’s. i think some are made from mold, but plaster of paris would be to ‘weak’ it would break with hammering. also, how to make ‘die’ out of metal to use in forming copper. where would i get information on this technique- also, different ‘washes’ to color and protect copper. kind thanks, frank
artydash :
Date: August 9, 2011 @ 11:33 am
Sorry I don’t have this kind of information. I only try and demonstrate basic techniques. This sounds very advanced.
Diane