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Umm....this looks like it'll do just as much, if not more, damage to your hand, as it does your target.
If you dip your molds into soapy dishwater right before pouring in the casting medium, the soap residue will help reduce the surface tension and make it easier to tap/vibrate air bubbles out of the nooks and crannies.
ROCKITE brand Anchoring Cement is a very good casting medium for craft projects like this. I use it for casting wargaming terrain and dungeon tiles. It has no large aggregates in it, blends to a smooth milkshake-like consistancy, is more durable, and doesn't have those tiny pieces of carbon that make it hard for paint to adhere, like QUIKRETE has.It does cost a little bit more, but the improvement in casting quality and strength is very much worth it.
If you're doing a short stack like is pictured, this ought to work well enough......but if you try doing a larger amount, such as enough to fill a whole roll, in a single stack, you'll likely start to get inaccuracies. The longer a coin is in circulation, the more worn and thinner it becomes...nickles seem to be the most prone to this, but you could easily have a coin or two extra in your stack, of any denomination. Admittedly, for pennies, most people won't be bothered by a coin or two extra.....but with dimes or quarters, it'll add up quickly.
Right before you pour the cement in your molds, try dipping them in a bowl of soapy water (just plain old dish soap), then shake the excess off. The soapy residue will reduce the surface tension, and make it easier to tap all the bubbles out of the nooks and crannies of the casting.
I make minis and modular dungeon tiles in a similar way as this. I've used Quikrete before, and haven't been all that happy with the results. Rocktite Expansion Cement seems much better for replicating the fine detail...and is more shatter-resistant, when dropped. Give it a try sometime, you might just prefer the results!
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