Duke,
My take on the use of ceramic in your case is that it is permissible. Reason is that you ARE backgouging the second side, now it would be a different case if you did not address the second side(being it was welded from one side only). See Table 4.5(33) states....
" The
omission, but not the inclusion, of backing or backgouging. (all of the processes apply)
Also see the 5.10 Backing states....
" Roots of groove welds may be backed by copper, flux, glass tape,
ceramic, iron powder, or similar materials to prevent melt through......"
HTH,
John Wright