Ex igne vs. Ex igni

Hello,

I am new to this forum. I am interested in latin language but not an expert by any means (my latin classes are very far behind me).
I am a silversmith and wanted to use the expression "by fire"/"from fire" as part of a sub-title for a series of cast metal pieces.

Would "ex igne" be the correct expression or would it be "ex igni"? I see it written both ways in my research and I want to be as correct as possible.

thank you very much.