Is it a 1% chance every time you try and upgrade a weapon or armor enchant from each rank? No 10% or higher, from shard, to rank 7, 8, 9 each time is a 1% chance...so basically you need a Coal Ward every time unless you want to play the ARC Casino that is Pres wards and waste several hundreds? Would explain why they are soo expensive on the AH.
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You can make a good guess where the Coalescent Wards are located, and Tradebar Store (technically) doesn't count.