STOP! Never, ever attempt to iron a patch onto leather! You are only asking for misery. Most leather used in leather jackets or vests cannot withstand the heat generated by an iron. Pressing the iron onto the leather (even with a t-shirt or pillowcase in between) will burn and scar the leather. You do not want the shape of an iron on the back of your jacket.
HOT IRON + LEATHER = PERMANENT BURN MARKS!
You may be saying to yourself, "But I've heard that the highest quality leathers can withstand the heat of an iron." This may be true - the only problem is that leather good enough to withstand a hot iron is also good enough not to let a little hot glue on the back of the embroidered patch stick to it. So, you won't burn the leather in this case, but the patch won't stick, either.
If you have some patches that you need to put onto leather, don't make the serious mistake of attempting to iron them on! Instead, see our page filled with Sewing Tips & Tricks to find out how to easily and professionally apply your patches to a leather garment.
TIPS & TRICKS FOR SEWING PATCHES ONTO LEATHER
IRON INSTRUCTIONS FOR DENIM and other IRON-SAFE MATERIALS
BUY MORE PATCHES FOR YOUR JACKET OR VEST