First an answer to your question...
RE USING OLD CANDLES
You can remelt candles and repour them into moulds or new containers.
Make the same kind of candle that the old wax was.... that is, if you have old container candles, use the wax for new container candles. If you have old free standing candles (moulded candles) use the old wax for new moulded candles. This way you will probably have the correct melt point wax for the type of candle you are making.
Melt down the candles, remove debris or pieces of old wick. You can strain wax through a nylon stocking stretched over a metal container.
Here is a link for you to learn about candle making.....
http://members.iinet.net.au/~campbell1/projects.htm