Mark A. Dyer on thu 30 may 02
Hi John
Thanks for your response. The cobalt that i have is a lavender powder.
It is from a batch that I just purchased. I have tried increasing the
amount of cobalt by a 1/2% up to 2%. I also tried lowering the RIO by
1% each time to 7%. My test were as follows
cobalt 1%RIO 9% cobalt 1.5%RIO 9% cobalt 2%RIO 9%
cobalt 1%RIO 8% cobalt 1%RIO 7%
cobalt 1.5%RIO 8% cobalt 1.5%RIO 7%
cobalt 2%RIO 8% cobalt 2%RIO 7%
Not knowing the purity, IYO could I push the cobalt up and still be safe?
On a side note the book has been a great resource for me and you both
have my many thanks.
Thank you
Mark
On Wednesday, May 29, 2002, at 10:05 PM, John Hesselberth wrote:
>
> Hi Mark,
>
> I could only guess that you do not really have cobalt carbonate in your
> bag
> labeled as such. Is it lavender powder in the bag? If not that is your
> problem. You may also be applying it too thin, but I'm most suspicious
> of
> your cobalt--I suspect it is either not cobalt or it is of very, very
> low
> purity.
>
> Regards,
>
> John
>
> web sites: http://www.masteringglazes.com and
> http://www.frogpondpottery.com
> EMail: john@frogpondpottery.com
>
> "It is, perhaps, still necessary to say that the very best glazes cannot
> conceal badly shaped pots..." David Green, Pottery Glazes
>
>
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John Hesselberth on fri 31 may 02
on 5/30/02 11:16 AM, Mark A. Dyer at ddmg@ISERV.NET wrote:
> Not knowing the purity, IYO could I push the cobalt up and still be safe?
> On a side note the book has been a great resource for me and you both
> have my many thanks.
> Thank you
> Mark
Hi Mark,
You would have to test to be sure it wasn't leaching, but, for an
experiment, I'd run a line blend from 0-5% cobalt. This glaze should be
near black at 1% and black at 2%. There have been occasional reports of
really low purity cobalt around, although I haven't heard of any for 3 or 4
years. I'd also order a pound from another supplier. While the vast
majority of our potters' supply chain are honest people, the price of cobalt
presents a big temptation to "water" it down. Someplace, among your
materials, you must have one that is not what it says it is--cobalt is just
the most likely suspect given the results you are getting.
And don't forget to try putting it on at various thickness' also.
Regards,
John
Web sites: http://www.masteringglazes.com and http://www.frogpondpottery.com
Email: john@frogpondpottery.com
"The life so short, the craft so long to learn." Chaucer's translation of
Hippocrates, 5th cent. B.C.
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