[Clayart] Clayart Digest, Vol 11, Issue 82

Jeff Lawrence jefflawr at gmail.com
Sat Oct 29 11:24:56 EDT 2016


>
> David H wrote:
>
> I have never heard of anyone calcining rutile.
> Pinholing can be a problem with rutile above 5 or 6%.
> But, it is equally a problem with titanium dioxide used
> in these percentages.
> Therefore, logically, I have always considered that titanium
> is the cause of the tendency to pinhole, not the organic
> matter in the rutile.
>
> Hello David,
Those pesky clustered pinholes in rutile blues always seemed due to the
inconsistency of rutile vs titanium dioxide, but I hadn't thought of them
as organic. My sense is they're non-rutilean rock crystals that resist
melting until temps where their decomposition products can cause maximum
heartbreak. I read somewhere (Parmalee?) that calcining the rutile broke
down some flourine compound in it. In fact, I did see a lot less pinholing
after bisque-firing my rutile to somewhere in the 06-04 range.
I share your logical pain, though. If bisque temps are what's required, the
ramp at higher temps to c10 ought to be ample time for tramp crystals to
break down before the melting glaze traps their flatulence. Also, some
pinholes still occur. Maybe a few stubborn gassy grains are too large to
submit to the first heating?
hmmm, I say!
Jeff
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