A little simple information that can save time, money, worry

These are a few small things I didn't know when I started out molding and casting. Perhaps sharing them will spare someone else a little anxiety and money.

Silicone mold-making tips:

1. Clean silicone adheres to itself, and it's possible to add more silicone to a mix you've started. So, if you come up short in your estimation of how much silicone to mix for a mold, just mix up some more and pour it in. Everything will set up fine as long as it's the same kind of silicone. You don't have to wait for the first batch to cure before adding more, but it will work either way. Just make sure no mold releases, dust or other materials get on the mold in progress.
2. Clean, well-mixed silicone can also be recycled into itself, and this can go a long way toward stretching the expensive material. If you mix too much silicone, just let it set up in the bowl and then cut it up into small blocks with a knife. You might even put it through a strong meat grinder  (not something you plan to use on food) to make smaller chips, or what we call "croutons" at work. Then, next time you make a block mold, simply sink some of the recycled material into the new mold in deeper areas after pouring rubber. My boss likes to mix the croutons into the mix while we're making it.
3. It's also possible to recycle old molds. When a mold is worn out, give it a good cleaning with rubbing alcohol or mold cleaner, then cut it up and use it as described above.
4. Temperature makes no difference in silicone setup. Silicone will cure in heat and cold. However, tin-based silicones (the most common) do require humidity to cure, and cold air is drier, so it may take silicone longer to set up on a cold day. It's possible to speed up the cure time by adding humidity to a room with a humidifier or vaporizer.

Resin casting tips:

1. Casting resins heat when they set up. The more resin in the batch, the hotter it will get and the faster it will set up. Small, thin batches take longer to harden than bigger batches. Bigger batches, because of the heat, seem to shrink more. I've seen large castings pull away from the mold a little toward the end of the cure time.
2. You have more working time if the resin is colder when you start using it. However, don't allow it to get down to freezing temperatures.
3. Resins do adhere to themselves, so it's possible to add more resin to cured resin. You get the best adherence if you do it soon after the first batch cures. Basically, this means that if you're casting a piece and come up short in the amount of resin you mixed, just let the piece set up completely and then add more within a few minutes to top it off. However, because of the shrinkage mentioned in the first tip, this won't work well if at all with very large solid castings. The new resin will leak around the area where shrinkage has occurred and you'll have a resin "skin" on your casting that you'll have to try to peel off.
4. Do not add fresh liquid resin to a partially cured batch of resin. The fluids will mix together, throwing off the chemical reaction. The casting will look like it has set up, but resin "bleeding" is likely to result, in which fluid oozes out of the casting for months or longer.
5. It may be possible to "shock" a bleeding casting into curing. Set it out in the sun for a few days or submerge it in boiling water (don't use a kitchen pot!) to see if that works.
6. Recycle bits of cured resin by submerging them in new castings. Just take broken bits and sink them into the liquid resin to stretch the material. I use bad castings and, particularly, pour spouts this way.

The recipe for rocket sauce, an inexpensive, effective release for silicone molds

I made a big, big mistake the first time I made a complex two-part silicone block mold. I used an inadequate mold release. When you pour fresh silicone on cured silicone without properly applying mold release to the cured silicone, you end up with an even bigger block of silicone, practically inseparable, with your original sculpture, fossil or whatever trapped in the middle.

I thought what I did with that block mold was sufficient. I finished one side, turned it over, then used the parfilm release I had, a spray that is fine for making castings but not adequate to prevent silicone from adhering to silicone. What I should have done, and what I did do for the next several molds I made, was rub on a thin layer of petroleum jelly (Vaseline).

When I went to work for the paleo lab, I discovered "rocket sauce". Other shops call it "slime" and probably a few other names. It's a simple combination of petroleum jelly and mineral spirits or naphtha, easy to make, easy to apply. Nowadays I generally use it only on silicone, but it's also useful to rub a little, very little, into particularly tight areas on molds and even onto the surface of some fossils.

Mold makers and casters make note: Rocket sauce is only for silicone rubber molds. Do not use it on latex molds, ever. Petroleum jelly breaks down latex, which damages the mold.

The ingredients: Petroleum jelly (generic brands are fine),
mineral spirits or naphtha, and a sturdy jar.

Put petroleum jelly into the jar, followed by mineral spirits
or naphtha. The higher the ratio of liquid to petroleum jelly,
the thicker the rocket sauce will be. I like it to be kind of thin,
so I've made this batch at about a 50-50 ratio by volume.
The mixture is going to be heated, so don't fill it to the top
or it may spill over when it expands.

Place the mixture on a warming surface, while the surface is still cold.
 I'm using a hot plate that can be set at low heat. It also works
well to put the jar into a small pot of water, which can then be placed on
a gas flame. Make sure the jar's lid is off so that pressure cannot build up.
Turn the heating element to its lowest setting to start out; you can turn
it up later if the mix takes too long to liquefy (say, more than half am
hour). Setting the temperature too high could break the glass, as could flame.
 

Petroleum jelly will get thin as it warms up.

It helps to occasionally stir the mixture. I'm using a toothpick.

The mixture combines into a yellow liquid when it's hot. Turn  off
the heat but don't touch the jar until it's had time to cool.

The rocket sauce will thicken as it cools. This batch was still warm.

Always label your chemicals. I used a Sharpie on the jar.

Put on the lid after the rocket sauce mix cools to prevent the mineral spirits or naphtha from evaporating. Apply it to molds with a brush or fingertips.

Two useful ways to recycle milk jugs

As I write this in late summer 2013, one of my regional
grocery stores sells half-gallons of milk for 99 cents,
while full gallons are $2.99. Naturally, we buy mostly
half-gallons.

Want to take a bite out of your molding-and-casting budget? Plastic pitchers and paint trays can add up to a decent chunk of change every week. At the paleo lab where I work, we mix casting resins in plastic containers and those will become an unusable mess within a few days. If we're pouring a lot of molds, it's possible to destroy several containers in a day.

If I were paying for those myself, I'd turn purple if I had to throw away four or five pitchers a week. Even the cheap ones are going to be a couple bucks apiece. A much better solution is to take scissors to gallon or half-gallon milk jugs.

At the lab, we mostly use gallon jugs. At home, where I generally don't have to mix as much material at once, I prefer half-gallon jugs.

Turning a milk jug into a pitcher or paint tray is easily accomplished. First, fill the jug about a third of the way with tap water, shake it up, then pour it out. Do this two or three times, then let the fluid drain from the jug. Dispose of the lid and take up a pair of sharp scissors.

Making a milk-jug paint tray

I use acrylic paints for most of the work I do both at home and at work. Some of it is good stuff, such as Liquitex, but more of it is basic craft acrylic paint such as FolkArt, Anita's or Delta Ceramcoat. Either way, when I brush it on I generally put a little paint on a plastic tray, which is easy to clean up with water when I'm finished.

Craft stores sell neat little trays with circular indentations for the paint, and they last a long time. Still, I've come to prefer this alternative.

The red Sharpie line shows where to cut the jug.

Cut with sharp scissors.



You end up with two trays, one of which has a palette-like handle.
They're easy to make, easy to use, and cheap to replace if you decide
it's gotten too messy to clean up.

Making a milk-jug pitcher

If what you want is a container to hold the casting resin, all you need to do with a milk jug is rinse it out well and then let it thoroughly drain before filling it with resin. Neither milk nor water mix with resin, so make sure it's dry.

However, the top of the milk jug is much too narrow to be easily used to mix resin and pour it into a mold. All it takes to make it workable is to cut out a little bit of plastic.

The red Sharpie line shows where I plan to cut. Adjust as desired
to make the pitcher either shallower or deeper. Don't go too shallow,
though, or the pitcher will be flimsy.

Here's what you end up with after the jug is cut. There's probably
a use for the little bit of plastic at right, but I couldn't say
what it is so I dispose of it.

A quick pinch of the plastic where the fluid will pour makes the stream
tighter, easier to control.

The final product. It's useful for mixing casting resins or for a variety
of other purposes.