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Home > Tomes of Wisdom > Building Middle Earth >Terrain Foundations > Casting the Round Fort

[Printableversion]

The time now is Wed Apr 15, 2009 1:53 am

DescriptionAn explanation of the process ofmaking a latex mould and castingwith plaster

Author Natarn Ulka

Date

Sun Apr 12, 2009 10:40 pm

Category Terrain FoundationsType Technique

Complexity

Advanced

Viewer Comments [6 - Post your comments]

Casting the Round Fort

An explanation of the process of making a latex mould and casting with plaster

Casting the Round FortAn explanation of the process of making a latex mould and casting with plaster


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In total there are 17members online :: 4Registered, 0 Hidden and 13Guests

Online Members: Erunion,Gorthaur, Valdor,whafrog

Introduction

Having spent countless hours sculpting my original fort, I thought I'd spend countless more

making a mould of it so that I could quickly make copies by casting in plaster. I admit that I

didn't realise that it would take quite so long...

This is not a particularly complex process but it is certainly long-winded. Was it worth it? Well,

judge for yourself with the finished ruined round fort. The painted fort in my Ruined Round Fort

article is not the original sculpt: it is one of my casts.

If you ever have the crazy notion of making a latex mould, then this article will certainly be of

interest to you. If not, well, I hope you find it interesting anyway.

Materials:Acrylic PaintBoardPlastercineLatex (liquid rubbersolution)Latex thickenerModeller's castingplasterWaterSand

Equipment:Face/dust maskJam jars with lidsOld Large/mediumbrushesBellows (a bike pumpshould do)Measuring jugScalesOld spoonsPlastic bucketsOld washing up bowlStirring sticksOld washing up brushModelling knife

This was the largest tub of latex I could find; it contained 5

litres. The larger the quantity, the cheaper the unit price. I

didn't know how much I would need so went for bulk. Iestimate that I used only 1 litre to make two moulds of the

fort, so I've lots of latex left for future projects. Far smaller

bottles of latex are available.

This tub cost about 16 so one fort mould cost about 3 to

make. I ordered the latex online through eBay. The

company selling mould-making materials is called TOMPS.

[Tue 18:32]Dorthonion: I havespent hours trying toget a blastedapplication for my jobto work... completewaste of time

[Tue 18:36]lorderkenbrand:painful, but leastyou're on here now,much better :)

[Tue 18:39]Dorthonion: Yeah...but if I get calledduring the night, itmeans a journey tothe office...:(

[Tue 19:49] Corsair:Hey Captain - I justmissed you in chat -care to go back? :D

[Tue 19:59] CaptainIngold: I'll be there,again!

[Tue 20:01] Corsair:

Send Refresh


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The latex thickener cost 1.25 and it was a 100ml bottle. The

bottle label shows the amounts needed and I started cautiously

but ended up just tipping it all in to my large jam jar full of

latex!

I bought the thickener from a local pottery supplier but it can

also be ordered through TOMPS online.

Ultramix is a great casting plaster for models. Again, I went for

quantity to reduce the unit price but, of course, smaller bags of

plaster are available. This one was 25Kg I think, and it cost

about 27. Each cast that I make uses 2 worth of plaster.

The pottery suppliers I went to decant large bags into 500g or

1kg bags and hike the price per unit. If you are lucky enough to

have a local potter's supplier near you, then it's a good idea to

purchase locally. The same type of plaster can be bought online

but of course the postage adds to the cost.

Work area and precautions:

a) Make sure you work in a well ventilated area. Some of these materials, such as the

ammonia (an ingredient of the latex) stinks. Inhale too much and your lungs will rot.

b) Don't breathe in plaster dust. Over time, a build up of dust in the lungs can cause

silicosis. Make sure you use a dust mask while you work with it.

c) These materials can (and will) stain or scratch easilly. Make sure you don't forget to

protect the surfaces of your working area.

1. Preparation of the model

Paint the model with acrylic paint. This stops the latex sticking to it and ripping pieces off when

you demould. Next, mount the model on a board and build up a rim of Plastercine. This will

increase the size of the mould and will act to catch any plaster overfill.


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2. Applying the latex

Heart-in-mouth moment - painting latex over my model! What if it doesn't come off? Will my

terrain piece be utterly ruined after those mind-numbing hours of sculpting putty?


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The first coat needs to be applied thinly and with great care. After all, this is the coat which will

capture the surface details of the model. It is essential to take one's time and do this bit right

(even if you might be tempted to be a bit sloppy later).

Brush from the top of the model and allow the latex to flow into the crevices.


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3. Avoiding blemishes

Where's the fire? Why the bellows? Air bubbles, is the answer. By massaging the thin layer of wet

latex with a stream of air from the bellows, the air bubbles trapped in the latex get blown out.

This prevents air bubbles spoiling the first few layers. I imagine a bike pump could perform the

same function.

Now comes the boring bit - and it lasts a long time. When the first layer has totally dried, apply

a second layer. It is obvious that it has dried because the colour of the latex changes from white

to golden yellow.


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When the second layer has totally dried, apply a third layer.

Once three or four layers have been applied it is good to add latex thickener, which will speed up

the process (but not much). This product does what it says on the can. Just add the amount

directed to your latex and stir. The latex then becomes more viscous and will stick to verticals

more readily.


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Latex thickener is useful because it means the thickness of the mould can be built up more

quickly. The downside is that drying times between coats are extended. So, it means you have to

apply fewer coats but the time the project takes may be just as long.

The application of latex for this one mould took about a day and a half! I worked on a hot

summer's day and took the mould out into the sunshine to dry after applications. It is not advised

to heat the mould with a hairdryer because it can cause the latex to vulcanize. This means that

the layers bond and any subsequent layers will not bond, but will peel off once dry.


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In the evening of day one, I gave it a thick coat and put the mould in a cool place to slow the

drying process. This allowed me to continue the next day.

The following picture shows a dry layer. Note the golden colour. The mould is obviously

thickening well and is becoming more opaque.

Eventually a thick enough layer was built up.


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It ended up looking like some kind of weird cream cheese.

A mould should have a thickness of about 5mm. The thicker the mould, the less likely it will be to

deform when casting. However, it is harder to demould if it is very thick and delicate details can

be broken.

4. Demoulding

Demoulding the original model is almost as magical as demoulding a cast. It hasn't been

destroyed! My model's okay!


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I left it about a week before demoulding (showing amazing self-restraint on my part). This gave

the mould time to vulcanize in a warm room. Some people advise putting the mould in a

cardboard box for a few days to ensure a constant temperature, which aids the process of

vulcanization.

Peel it off with care to avoid tearing. The latex is tough but needs to be treated with a degree of

respect.


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The inside of the finished mould.

The level of detail picked up by the latex is extraordinary.


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5. Preparing for casting

To limit deformation caused by the mass of the plaster in the mould I chose to support it in a bed

of sand.

The sand needs to be damp in order to hold its shape. I pressed the original model into the sand

to make a depression ready to receive the mould.


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The mould in place.

6. Mixing plaster

There is a wide variety of types of plaster. Polyfilla is great for rough and ready terrain such as

cliffs but doesn't quite cut the mustard when it comes to casting.

The best quality modeller's plaster is a very fine powder which helps retain detail. It is also

relatively strong when compared with cheaper options, being far denser.

It is important that the correct ratio of plaster and water is used for every cast. Too much water

and the plaster will be crumbly when dry. Too much plaster and it will not pour smoothly into the


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mould. Ratios are available from suppliers and are often marked on packaging. Scales and

measuring jugs are essential to maintain a good casting standard.

To avoid wastage, it is a good idea to work out the approximate volume of the mould by

measuring the amount of water needed to fill it. Just make sure that the mould is dry before you

make a cast.

7. Pouring plaster

The plaster should be poured while still in a 'liquid' state. Do not pour it all around the mould to

try to cover it because this can cause deformation and air bubbles. The plaster should be poured

slowly in one place and allowed to flow into all the details. As it moves it expels the air.

I do not pour the plaster onto the latex itself but place a small object in the mould and pour the

plaster onto that. It runs over the object and into the mould, spreading from that central position.

An upturned spoon is good for this.

When the mould is full, bang the table to remove any air bubbles. Banging the table causes

sufficient vibration to dislodge air. Do not bash the sides of the mould - this usually just creates

unnecessary mess.

Wait until the plaster feels hard before demoulding. It is also a good idea to wait for it to cool offa bit (plaster becomes warm/hot as chemical reactions take place). If a cast is demoulded too

soon the plaster will not have made sufficient bonds and will be likely to break. Remember, the

cast is unlikely to be as strong as your original model.

Demould with great care, unrolling a little of the mould at a time. Now is a time to move slowly.

This (for me at least) really is a magical moment - peeling off the mould to reveal the new model.


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Although the model feels strong very early on, it will take a long time to fully dry. I advise

leaving it for at least two weeks in a warm room before undercoating. Commercial mould-makers

have drying rooms or cupboards with extractors to speed things up. Do not put your plaster cast

in an oven to dry it out. When plaster gets too hot, it explodes.

If necessary, trim the cast as soon as possible after casting with a sharp, strong knife. Now is

also a good time for adding extra details, scratching into the plaster with a sharp tool. The plaster

is relatively soft at this stage and cutting it will become more difficult as the plaster dries.

Conclusion

So, the big question: Is it worth the effort? I would give a qualified 'yes' to that. Through this


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project I learnt an incredible amount and really value the experience. If anyone would like a cast

of this fort then I can make one at the drop of a hat. Lord Erkenbrand and Tidoco are pleased

with theirs!

Casting using latex is certainly not suitable for all projects. If you want lots of precision including

straight lines and right angles, then you will be disappointed. If, however, your project is a ruin

such as this, then I think latex is ideal. People who have seen the cast have been amazed at how

closely the detail has been replicated.

I will use latex again to make moulds (I have four litres left!) though I will probably use it on

more generic terrain items such as walls or to add details to larger projects. In this way I can

make one highly detailed piece and copy it as many times as I wish, which will save time and

effort in the long run.

The finished fort with extended base, sand and flock.

-= Natarn Ulka =-

See also:

Ruined Round Fort - Natarn Ulka

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