When I couldn't make the concrete thin enough to transmit light I thought it might be interesting to do a casting where the concrete had stained glass set within it. Four separate panes of yellow stained glass are set, one in each side. As with the slotted candleboxes, this is 3" square and 3 and 11/16" high. The glass is true divided light, bitten by the concrete - it is not placed only on the inside.
During casting, one of the panes came loose from the mold and moved (not visible in this photo). And although it came out of the mold in excellent condition, over the next two weeks it began to crack, at first the glass, and then the concrete. The cracks aren't structural, but they do destroy the aesthetic of what I was trying to achieve.
A few years later I cast a second small candlebox with deep read mottled glass and the same non-shrink grout I used in the Test Panel. It came out great, but also cracked over time.