Tuesday, 22 May 2018

How To Make A Latex Mould and Plaster Support Jacket

Materials used:
Glass louvre window sheet
Spray on acrylic sealer
Old paintbrush for latex
Banjo and Milo, the Jack Russell dogs
A camera to take a photo of the bloody hole that the dog dug to demonstrate his mice hunting skills
Gypsum/plaster powder + tool for mixing, bucket, latex glove, smoothing tool
PVA glue, plastilene/plasticene (same thing really)
Silicone pastry brush (doesn't shed hair) & vaseline

I have made a latex mould featuring multiple two dimensional, flat back positive objects. 

First, my positive objects are glued on a sheet of louvre glass window, with glass smooth side up. Leave enough room for a latex 'collar' around each object. 

Then, I backfill any undercuts and gaps underneath the positive object, with plastilene or PVA glue...this is because a silicone filler will bond to the latex and we don't want the mould to be stuck permanently to the glass. With the clown and buddha above - gaps were filled in with PVA glue and trimmed. Acrylic sealer is applied to the top surface of all positives, so that the latex doesn't absorb any paint particles (which will affect any soap etc that is poured into the finished mould). You could also use clear nail varnish or a weatherproof paver sealer which is water based.

And now for some entertainment...  

Here's Milo the Jack Russell x Foxy, digging for mice....he's 12 yo now and his older buddy Banjo the Jack Russell is now 14 and has just retired from digging tunnels looking for mice. Milo is so proud of the hole that he has dug, he is barking in the photo below. What little champions at keeping the mice and rats at bay in my bird aviaries and around the house. Back in the day, Banjo caught hundreds during a mice plague, snapping their spinal cord, spitting them out, then onto the next one.  Milo, however was just starting out as a wee pup and he tried to eat all of the mice as he caught them, bless him...he got a bit tired from eating soooo many.

Ok, back to the moulding. 

Latex is white when wet and yellow once dry. Recoating with the next latex layer is necessary before the previous layer completely hardens, when it is no longer white. Re-coating wet latex with more wet latex just wipes off the previous layer...In between coats, I just leave my brush in the latex. When it clumps I wipe it off. Leaving brushes in white spirit or soapy water does nothing to preserve the life of the paintbrush...just buy a pack of cheap brushes from anywhere and toss them out if they get too clumpy in between layers.

A very smooth latex coat on the first layer of the original positive object is most important...the supporting 'collars' or any area outside of the detail of the positive object doesn't need to be smooth...lumps/bumps are pretty normal on the outside area /collar area of the positive.


Latex mould finished! After 15 layers, the mould is ready for a plaster support jacket. A silicone brush is used to brush petroleum jelly all over the mould, into every crevice.


A perfect plaster smoothing tool...a cake icing smoother. Not that I ever make icing for cakes any more, so I'll put it to better use.    

Wet plaster poured over the top side of the finished latex mould. Once it is dry, it is flipped over and becomes the bottom support in which the latex moulds sits inside of.  

It is easier to make a cardboard or vinyl support around the latex mould base, to pour the plaster into. In my previous blog, I talked about using short pieces of timber dowel as registration points for the mould, as a guide to the finished height of the wet plaster. The bottom of the plaster just has to be level enough for the latex mould to sit in...you can always file it down a bit with a rasp when dry. (As you can see, I don't practice what I preach...my plaster moulds are poured arty hippy freestyle without any structure to support the plaster). The plaster is quite soft but firm enough to be smoothed once over the top. It gives me enough time to tweak the plaster support a bit more if I need to, rather than pouring plaster which is too thick to manipulate/move around and has set up too quickly. 

I hope that you are now inspired to start making some moulds! 

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