What am I doing wrong?

[Lindyloo]

Before I tear my hair out, can some kind person help me. I have now managed to crack 3 crucibles. Here is a picture of the latest one. With the previous two, I got scared off before adding any silver (I had just put some borax in), and didn't continue with them. But with this being the third, I got fed up and just used it anyway, with no ill effects.

After cracking the first two, I spoke with Ben in the chat room, who suggested that it might have been because I kept them in the garage, and they were cold when I put the flame to them. So this time, I kept the crucible inside the house, then took it out, and waved the flame gently over it first to warm it up, before going in full blast. But still it cracked. What is wrong with me?