I use a lot of coconuts so I always have shells lying around. I hate throwing something out if there is another possible use for it, my family considers me a harder but I only keep the useful things.
I use young coconuts which have a much lighter shell than their mature counterparts. The young coconuts come still surrounded by the husk and need to be cut out before they can be dried and used for other things. Let the shell dry for a couple of days before trying to make anything with it.
Step 1: Cutting the Cups
I used a plastic cup with what I considered to be the appropriate circumference as my template circle. using a band saw I cut the bottoms off the coconut at the mark from the template. The band saw made it very easy as I was able to pretty much cut straight across the shell.
Step 2: Sanding, Sanding, and More Sanding
The shells need a decent amount of sanding to get a smooth finish. I start with 80 grit and step up with finer grits(120, 220 & 400) until I get to 1000 grit. 400 probably would have been high enough. Make sure to smooth the lip of the cup as much as possible.
Step 3: Oil
Once the shells were smooth, I worked a small amount of coconut oil into both sides(coconut oil is my preference, but any food based oil would work). The first picture shows the difference between oil(left) and no oil(right) on the outer shell. The oil is not really a necessary step but I feel better about the finished product if I do it.
Step 4: Bases
To create the bases for for my shells, I used a cordial glass and some 2-part silicone molding putty. Combine the 2 parts of the putty and form around the rim of the glass. The putty takes about 30 min to set and can then be removed from the glass.
I used acrylic resin from my local craft store to pour the bases. You do not need much resin to form each base, a colorant can be added if you want. I used very small amounts of washable acrylic paint so it would not keep the resin from curing properly. Don't be afraid to experiment with different colorants, the professional ones can get expensive so I rarely buy them.
I left the bases to cure at least 8 hours before un-molding them, it normally would not take so long but we have been having extremely humid weather which does extend the curing time. I only made one mold for the bases so it took 2 days to cast all 4. Once the bases were finished curing and removed they needed to have any imperfections trimmed away, I used a sharp utility knife. The edges were smoother with a sander.
Step 5: Putting Them Together
After the bases were finished I used a food safe silicone glue to attach the base to the shell. I leveled the shell as much as possible, the coconut shells are not perfectly round so your cups are not going to be perfectly round or level. Carefully wipe away any excess silicone from were the base meets the shell, I wanted them to be as clean as possible. Once all four shells we're attached to their bases they were left to dry overnight.
Step 6: Drinks!!
I wanted to make sure that leaving the skin in the shell was not going to cause any problems with the cups if acidic liquids were added. I filled the cups with different drinks; orange Juice(top left), white wine(top right), brandy(bottom left) and sake(bottom right). The liquids were left for 2 hours in the cups with no problems.
Next time I make another set I might remove the skin to see the effects different drinks have on the shell directly, but that will take more sanding than I really want to do now.