It’s unlikely you’ll need to do much to prepare your molds in any special way. Silicon molds can be turned inside-out to unmold your finished chalk. Regular muffin tins can be turned upside-down and given a good thump. Cardboard containers used as molds can simply be peeled off of the finished chalk.
To make a nice, big chalk stick, seal one end of a toilet paper tube, a cut-down paper towel or wrapping paper tube, or the long, narrow box that a tube of toothpaste comes in with duct tape. To make other shapes of chalk, search your recycling bin for other likely containers–if the container can be ripped or cut, then you’ll be able to unmold your finished chalk just fine when it’s set.
When you have your molds and your plaster of Paris mixture ready, simply pour the dyed plaster into each mold, thump the mold onto your work surface a few times to release any bubbles, and let it rest until it’s set, which will likely take longer than indicated by the directions on your plaster of Paris package, since you’ve added in that pigment.
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