Making a Simple Rock Mold with Silicone Caulk

[Jknuth]

Ok well we will give this a shot.
I will demonstrate the techniques i use for molding simple objects using silicone caulk.

Things you need

1. silicone caulk (it must say 100% pure silicone) I use clear, but in a bind others colors will work
2. Dish soap (dawn)
3. small disposable brush
4. acrylic paint, any color will work but i use white or bright green.

first Ill explain the properties of silicone caulk.
many think it is simply air dry like most paint. this is not the case. It is condensation cured silicone, meaning water is its catalyst.
if you put it on to thick it will cure on the outside due to moisture in the air. but it will form a pocket of liquid silicone that can last years.
since it is impossible to add water to silicone by itself due to the way liquid silicone repels water. I have learned to use acrylic paint as a fast curing catalyst for caulk.
With this method it is possible to mix an entire tube of silicone with a few drops of paint and from a ball the size of a softball that will set in 45 minutes, thus eliminating the "thickness" issue.

Another issue with silicone is its adhesive properties. It is one of the stickyist substances in its liquid form. once it is cured however there are few things that it will stick to.
this is why when working with liquid silicone I use dish soap diluted with water to move the silicone around.

A few notes before i get started.
Like i said above silicone does not stick to much, but it will adhere to other Silicone, occasionally glass. Some rocks do contain natural glass and the silicone will stick to it.
I recommend using petroleum jelly to oil the rock before you begin.

Ok so we begin

first grab a rock

 

[Jknuth]

next Brush on the silicone.
for the first coat i use PURE silicone out of the tube. we are making a thin base coat to hold the detail.
I use a brush to push the silicone into the cracks and crevices of the stone.

[Jknuth]

after the stone is covered in a thin layer of caulk we will work on smoothing it out, and forcing any air out.

[Jknuth]

next step mix up a cup full of dish soap and water half and half.
here is where it is interesting. Dip your fingers in the dish soap solution. Be sure to cover your fingers and hands in in the soap.
as long as your fingers are wet they will not stick to the silicone.
and remember not to worry about water on the surface. it will actually help it sure faster.
begin smoothing your fingers over the surface of the silicone. Force out any air and make sure it is a uniform thickness.
for this mold it is less then an 1/8 of an inch.

[Jknuth]

finally you will end up with a stone with a smooth layer of silicone over the surface. let this layer set before you work any further, This is your detail coat so you dont want any gooey pockets in there.
I am going to let this sit over night and start up again in the morning. but in most cases it wont take longer then 2 hours to set. perhaps much less.

[weasel]

WOW thank you very much i got a project i'll need this for in upcomeign snake mount i love thsi forum because through the few scammers there tons and tons of nice people willing to share thier knowlede.
just wanted to thanks you,
colt s

[Jknuth]

Aghh forget sleep I want to see this through

next step.
thickening the mold
this is where it gets fun
add a small amount of acrylic paint to a cup full of silicone. just a drop. Unfortunately there isn't a hard formula for this, as it depends on temperature and humidity.
It will set FAST! faster then you think

[Jknuth]

Mix it to a uniform Booger green
and get ready!. did i mention it sets FAST!

[Jknuth]

next scoop it on top of the first coat of silicone.
the bright color helps in 3 ways.
1. making sure your mix is thorough
2. you will never loose another mold in the shop
3. it just looks COOL
 

[Jknuth]

finally smooth out the silicone blob.
use the same technique as before soapy hands, work out any bubbles and make sure it is thick enough not to tear on you.
Silicone caulk is far from a permanent mold so you have to take care of it and dont be too rough. although the tube of caulk cost me $2.50 and i havnt even used half. so thats not bad for a mold. plus in the middle of the night if you want to make a mold and dont have any good silicone left you can go to walmart and get ready to mold a rock on a whim. Now who in their right mind would do such a thing??
MMMMM shrek nugget

[weasel]

any acrillic paint will do right i got some leftover "kings gold" walmart 50cent stuff..... anyhow ever do more coats than that? or is it really not needed...especially on large set of horns and such.

[Jknuth]

any acrylic paint will do. I have made it thicker but it isn't always necessary. I have not molded horns yet with this method, but i see no reason why it wouldn't work.
 

[Jknuth]

well here we are. the mold is done
and I am about to demold.
just flip over the rock and begin to slowly pull off the mold, work slowly to make sure it isnt sticking anywhere. Silicone caulk has a relatively low tear strength so move slow here.

[Jknuth]

and there ya go mold removed.
The whole process took about 2 hours. with actual  work time of about 15 minutes. cost around $1.75
mold will last many pulls. and will outlast latex.

[Jknuth]

I cant seem to stop here
nest steps fill with your favorite resin. I used thinned bondo and just rolled it around inside until it set up.
a real shell casting.