Well, my wax that I use for sewing is a real antique.  I will bet it is over 50 years old!!!

Here is my tip:

• Take a smaller glass (a juice glass or paper cup is good) and put about 2" of water in it
• Place your wax into the glass, it will float (duh)
• Place in your microwave for no more than 30 seconds (learned that the hard way - still cleaning wax off the inside!!)
• Check to see if the wax has melted, repeat with less time if needed
  
• Do not over heat the water, you will just have a mess
• Let the wax solidify but not completely harden
• The bottom will be filled with years of unknown debris, use a knife or paper towel to remove it

You have a new, clean piece of wax. 

NOTE:  I learned these steps because I did exactly the opposite on each step . . . .

Pat, The Avid Embroiderer

Mine is so old, I have no idea.  But any wax would do the same.  Mine is a light colored yellow/orange. 

Pat, The Avid Embroiderer