When an alchemist makes a "Major XXX Protection Potion" (you make a batch of 5 at a time, and it requires primals), then you have a fairly high chance of discovering the recipe for a Cauldron of Major XXX Protection".
When I say "fairly high", it seems that making anything between 1-5 batches will commonly allow you to discover the cauldron. My stats have been:
Arcane: 3 batches = discovered cauldron
Frost: 3 batches = discovered cauldron
Holy: 12 batches, no discovery *** See warning below
Shadow: 3 batches = discovered cauldron
Nature: 2 batches = discovered cauldron
Fire : <I dont have the recipe for Major Fire Protection Potion
*** Regarding Holy; after using a lot of mats trying to do this, I decided to check forums etc; it's unclear whether a Holy Cauldron even exists, so suggest you save your mats, wish I had
Hopefully the Nature and Frost cauldrons especially should be useful to us once we're in SSC (they allow you to make 25 protection pots from roughly the same level of mats that would have been required for 10 pots normally).
As.