about moulds

Tuesday, 26 May 2015

Tutorial: Making paper plaster mould

I use this method to make larger moulds without cottles. It creates lighter, stronger and less bulky moulds than traditional plaster technique, and requires less preparation time.  

Materials and equipment:

- model
- mould release (I use soft soap or watered down clay slip)
- containers for mixing and measuring
- scales
- plaster (I use Gyprock Superfine Plaster)
- water
- paper pulp ( available in bulk from roof insulation places)
optional: kitchen timer

A word about undercuts:

Undercut is any indentation or protrusion in the shape that will prevent it coming out of the mould. You have to be careful to prevent undercuts when making even the simple drop - out mould, and even more so with multiple piece moulds.
For example, if you are making a 2 piece mould of a sphere, dividing it on any point but the widest will create an undercut and prevent your object from coming freely out from the mould:

1) Preparing the model

The model needs to be non-porous. I am using a cardboard model, put together with sellotape and  covered with remnants of sticky back plastic.As I am making a 2 piece mould, I have used cardboard to create a barrier  between 2 halves. The barrier is 4 cm wide, so I will have the visual indication of the plaster thickness. It should not get wider than 4 cm.

2) Mixing the plaster

I always weigh the water and the plaster - that way all my moulds ( and parts of the moulds) have a consistent porosity. That is not really important when press-moulding, but it is essential when slipcasting. I just find it easier as it takes the guesswork out of the process.
Ratios that I use are 1000g of plaster to a 700g of water.
I estimate the volume of plaster that I will need in litres, trying to err on the generous side. I ask myself how many 1 litre milk cartons can I put in the space I intend to fill with plaster. In this case the answer is 2. 
So I poured 1.4 litres (equivalent to 1.4 kg) of water into a bucket, put it on the scales and added 2 kg of plaster to it. that will give me a rough 2 l of plaster in volume.
Rule worth remembering: Always add powder to the water.

- Add plaster to water (ratio 1000g plaster to 700g water)
- wait for the plaster to be absorbed 
- mix plaster & water into the smooth consistency
- leave it to thicken (maybe 10-15 minutes - that is where the kitchen timer comes in handy if you know the properties of your plaster)
- it is ready when your finger run across the surface leaves clear path
- mix the plaster thoroughly but quickly

3) Applying

Apply plaster from the highest point, letting it run down all the walls. Use your fingers to make sure that all areas are covered and there are no air pockets. You have only short time to do this, so work quickly.Aim for about 1 cm thickness. The purpose of this layer is to preserve all fine details of the model and absorb the moisture from the clay.
Because the plaster is on the point of stiffening when you start applying it, the moisture does not have the time to weaken the cardboard. 

4)Mixing the paper plaster

Usually you will need more volume of plaster for the second layer, as it is larger. 
Start the same way as for the first layer:

- Add plaster to water (ratio 700 g plaster to 1000 g water)
- wait for the plaster to be absorbed 
- mix plaster & water into the smooth consistency 
- add dry paper pulp while mixing, until consistency thickens and it becomes pliable.
- apply immediately on to the mould

5) Preparing the second half

 This is the point that I add a cardboard wall to the future opening of the mould. It makes it easier to build a neat plaster wall around the opening. yes, the photo is of the different mould, but the principal is the same. You can do this earlier in the process,but then you would be working on a higher base for the first half of the mould.

 Don't forget to make several round holes ( the smallest coin twirled around will do the job, or any roundish studio tools you can put your hands on). Those holes will be the "keys" helping to accurately lock two halves of the mould together.

 Apply mould release. Plaster will stick to plaster unless you create some sort of non- porous barrier. On this photo, I am applying a watered down casting slip. Sometimes I use soft soap (liquid soap) that I rub with the tips of my fingers into the plaster. If using soft soap, keep applying layers until it saturates the surface of the plaster.
I usually apply it to the sides of the mould as well as the top edge, as some plaster is likely to spill over the side. This will make it easier to clean

6)  Repeat steps 2, 3 & 4

And here it is, finished mould. You don't even have to wait fort he plaster to cool down. Gently tap it onto the ground on the seam and it should crack open like an egg. I love this moment.

I tend to let the mould dry for a few days before using it. If you are going to use it for slip casting, you will need to wait for a week or more as the mould needs to thoroughly dry out in order to absorb the water from the casting slip.
If it will be used for a press moulding, it can be used almost immediately.

Did you find this tutorial useful?
Is it similar or different to the way you make moulds? 
Do you have suggestions or tips to share?

I would love to hear your comments.

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