[Katherine Swift]

This question comes through email by Monica:

Hello,

I’m brand NEW to casting, this is actually my first week! I’m loving it, but I’m having problems with cracking and dullness.

I’m using “Castin’ Craft” Polyester clear resin with the matching brand catalyst.

Here’s my issue. I only want to mix very small quantities of resin at a time. I can’t seem to get a straight answer about how much catalyst to use for a half ounce of resin or less. I’m trying to cast TINY resin pieces for earrings and rings etc. Nothing big for now.

Here are the two things I’ve tried to make:

The first one is a heart shaped piece about the size of s 50 cent piece. I mixed 1/4 ounce of polyester resin with 3 drops of the matching catalyst. I used a heart shaped ice cube tray but only filled it up 25% full to keep the piece thin. I found out that latex is not recommended for molding resin. That piece didn’t crack but came out dull and frosty. I won’t do that again.

The second thing I’ve tried (four times now) is a 14mm marble. Same resin/catalyst ratio as above. I made the marble out of a rubber mold I made a few days ago. The marble comes out perfectly round but VERY cracked and dull EVERYTIME.

Here’s the ratios I’ve tried:

1/4 ounce with 3 drops of catalyst
1/2 ounce with 8 drops
1/2 ounce with 10 drops
1/2 ounce with 12 drops
(Less cracking with this last ratio)

Please tell me how many drops of catalyst I need for 1/2 ounce of resin??

Here’s more info . . . I live in Arizona and my house is about 77/78 when I cast. I’ve tried covering and not covering. . . Nothing helps. I want it to be crystal clear.

Please help! I appreciate any and all advice!

[Katherine Swift]

Hi Monica,

I’m happy to hear you are having fun with resin casting. Enjoy the journey!

Polyester resin can be tricky to work with and I generally don’t recommend it for beginners. When it comes to adding catalyst to the resin, you have to start with the end in mind. You add drops of hardener based upon the thickness of the final casting. How deep are your spheres? That will dictate how much catalyst you need. The ironic thing about polyester casting is that the thinner the casting, the more catalyst you need to generate enough heat to allow resin curing to occur.

The issues with your heart-shaped silicone mold is likely due to the mold. Castings will pick up the surface of the mold and impart them on the casting. If shiny templates weren’t used to make the mold, castings from that mold won’t be shiny either.

As for your castings, I think they are getting too hot and subsequently cracking. If you can tell me how deep your castings are, I can help you tweak the amount of hardener you use. Your room is a little warm (72 or so is ideal), which may also be impacting the heating up and subsequent cracking.

Have you considered an epoxy resin? I like polyester resin for larger castings, but based upon your description and the pictures you sent separately, I think getting polyester to work in this situation is going to be a challenge. (Plus the smell is horrible.)

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