This is fired in cone 10R. The effect becomes more intense by 5%. To achieve this same effect using Ravenscrag, which has much less natural iron content, 10% added iron is needed (which is, of course, much messier to work with).
L3341B - Alberta Slip Iron Crystal Cone 10R
By adding a little iron to 100% Alberta Slip you can make an iron crystal glaze.
Duplicating Albany Slip
How Alberta Slip was created by analysing and duplicating the physical and chemical properties of Albany Slip