Situation: I’m making a decoy cup of coffee. I have a standard porcelain coffee cup. I need to fill it with some sort of black plastic that will appear to be coffee. It seems like there should be some sort of pourable plastic-like material that I could add black dye to that would dry solid/hard.
I hope I’m explaining it well. Any suggestions?
Thanks
I have a quart of black potting epoxy.
I bought it on ebay.
ETA:
How about: http://www.ebay.com/itm/Colored-Epoxy-Resin-Kit-Black-HIGH-GLOSS-CASTING-TILE-SHOWER-REPAIR-/281581804150?hash=item418f955a76:g:azAAAOSw8d9Uyudk
That looks pretty promising! Will it cure no matter how thickly poured?
Epoxy and polyester cure by an exothermic reaction, a full quart left to cure will possibly catch fire, you need to fill the cup with something inert and put a small layer of resin on top.
Yikes! Thanks.
You can also use acrylic artist paint to color clear epoxy. I think you will be fine heat-wise mixing up 8 oz. It will get warm but it is contained in a coffee cup.
Fill the cup with untinted epoxy or even random candlewax or something, then add a 1/8–1/4" layer of the resin mixed with craft (acrylic) paint or pigment dye. Don’t dye the whole cup, or it might take forever to dry. Protip–mix your dyed top layer up with a whisk to get some bubbles in it, might lend a little more credibility. It will bubble better if you can add a bit of titanium powder (which will give you a cafe au lait look). as an emulsifier.
Depending on your prank, you could also go the route of using candle gel/wax. Or even (for a nontoxic one-time use) making a coffee gelatin with a ton of extra gelatin packets mixed in, so that your fake coffee is basically a plug of licorice/gummy. I don’t know what your goal is. 
Source: I’ve done a ton of this kind of stuff in the name of art restoration.
Some polyester casting resins are OK with larger volumes - typically the pack advises to add less catalyst and allow more curing time.
But yeah, seems a waste to fill the whole cup. Fill it two thirds full with gravel, then pour the tinted resin over the top - some of it will fill the interstices between the gravel, so it might need topping up a minute after the initial pour.
The gravel will fill volume, minimising resin waste. It will also cause air bubbles to rise in the resin - which, if done right, could add to the realism of the fake cofffee look.
If the prank and/or your time is worth $20 try this: Fake Mup of Coffee - Mug filled with Artificial Coffee - Props America
I’m not endorsing that particular vendor. It was simply the first of almost 5 million hits on Google for [filled fake coffee mugs].
I’m with **LSLGuy **- my grandpa (the cutest old man prankster) had a joke coffee cup and it didn’t look half as realistic as the one linked above, and it fooled us every time.
I’d fill most of it with something easier to work with, Bondo, modeling clay, a mix of plaster and foam beads to get the weight right, then top it off with a solid layer. There are places that make fake food for displays that get these things perfect like the props link above. Those little bubbles and slight variations in color can make the food look better than real.