For flat pieces of glass, simply scrape the wax off with a thin piece of plastic, fingernail or other non-metallic tool. For wax stuck in round or curved spaces, remove as much wax as possible, then place the item into hot water until the wax becomes warm enough to remove.
Using only warm water is key when getting wax out of a curved dish, bowl or cup. While boiling water seems preferable, its use presents dangers with certain types of glass. While products such as Pyrex are tempered to withstand high temperatures, many types of glass cannot withstand the heat of boiling water. In the worst case, the glass can shatter, destroying the item and potentially causing injury. For this same reason, it is vital to never boil water in the item or directly heat the glass.
With the wax safely removed, items such as candle holders have a wide variety of uses. The can, of course, be used to hold candles again, but there are other uses as well. The small bowls and cups make ideal places for storing needles, thumb tacks, pocket change or other small items. The larger jars make attractive decorations when filled with potpourri, colored sand or sea shells. However, candle holders should not be used to serve or hold food items even after cleaning, as the types of glass from which they are manufactured are rarely up to food grade standards.