What is causing the cracks?

[azure]

I made a batch of soy container candles the other day. The next day they had cracks in them. What is causing that?

 

[pepperi27]

I've heard it maybe because you did not pre heat the containers prior to pouring the soy into them? That's the case anyway for making glass container candles.

 

[CPSoaper]

It could be because they cooled too fast or if you added BW, it could be too much BW.

edited to add. I've never heated my jars and IMO feel it is a complete waste of time and totally not necessary.