Jeremy/Bonnie Hellman on sat 2 nov 96
Hello glaze wizards!
"My" crawling glaze is actually Chappell's floating blue glaze with
additional cobalt. I first tried this glaze after it was posted on Clayart
(before buying Chappell's book). The poster suggested that either cobalt
oxide or cobalt carbonate could be used, but in my haste to mix the glaze
(and on the theory that if a little blue is good, more blue is better) (ok,
get out the flame thrower) I added both. I screened it once through mesh 60
and got a beautiful variegated blue glaze with blue spots. I screened it a
second time and my spots disappeared (darn) so I've since screened it only
once through mesh 80, and get those great cobalt oxide spots. It gets great
reviews from most people and I like the way it looks. BTW where thin, it is
a dark brown. I fire it on cone 6 porcelain. When I fire it on Standard
Ceramics Stoneware # 112 which is manganese spotted light brown clay in
oxidation the glaze is a rich olive green with some blue where thick with
brown spots from the clay (I think). However, on my bowls (the ones that
hold about 6-8 cups) the glaze crawls a little on the outside near the top
rim--sometimes. Also it doesn't always crawl on all pots, and even when it
does crawl it does it in one or two or three places, each no larger than an
American quarter. I don't remember it crawling on my 1 cup bowls or on my
plates.
The glaze recipe I use (taken from Hyperglaze) follows, and I would
appreciate any opinions as to why and suggestions to improve it.
My firing schedule. Bisque to 06, Then glaze fire in an 8 hour cone 6 ox
schedule. Low for an hour, small 3 cu ft kiln top cracked open, 2 for
another hour and a half, top open until the smell stops. (Basement door is
open to the outside with room fan on to encourage air out the door.) Then 2
with top closed for an hour, then 3 on bottom switch & 2 on top switch for
an hour, then 4 on bottom & 3 on top for an hour, then 5 on bottom & 4 on
top for an hour, then 6 on bottom & 5 on top and it usually shuts off after
a half hour. It's a pretty even cone 6 firing maybe a little cooler than a
full 6.
Glaze name: Floating Blue-Chappell
Cone: 6
Color: Opaque Blue
Testing: Tested
Surface texture: Shiny or Glossy Semi-Gloss
Firing: Oxidation
Glaze type: ?????
Recipe: Percent Batch
Nepheline Syenite 47.30 2365
Gerstley borate 27.00 1350
flint (325m) 20.30 1015
EPK 5.40 270
Totals: 100.00 % 5000 gm
Also add:
cobalt oxide 1.00 50
red iron oxide 2.00 100
rutile 4.00 200
cmc (mix dry powders well)0.15 8
Comments:
1. Per Candice Roeder-use 2% cobalt carbonate instead (I've used both)
2. use premixed cmc & water-1 tsp (I use dry powder and mix well with other
dry powders before adding to water)
3. sieve 2-3 x thru 60-80 mesh- or sieve only once for mottled look
4. fire on light side or cone 5
5. may need to watch water (my tap water is OK--no difference when used
distilled)
6.if fired higher turns greener (yup-been there done that!)
Unity Formula for Floating Blue-Chappell:
0.100 K2O 0.554 Al2O3 4.074 SiO2
0.420 Na2O 0.485 B2O3 7.4:1 Si:Al Ratio
0.476 CaO
0.004 MgO
Percentage Analysis:
61.61 % SiO2
14.23 % Al2O3
8.49 % B2O3
2.37 % K2O
6.55 % Na2O
6.72 % CaO
0.05 % MgO
Thanks to Bob Kavanagh for the suggestion to post this. A bigger thanks if
the problem is solved!
Bonnie in Pittsburgh (who is willing to refire when necessary rather than
sacrifice this glaze)
>----------------------------Original message----------------------------
>Good morning to Bonnie in Pittsburgh
>
>Since you have a regular problem with your glaze (crawling at the lips),
>why not post the recipe (and perhaps your firing schedule) and see if
>someone can make suggestions about solving the problem.
>
>bob kavanagh (60 km west of montreal)
Craig Martell on mon 4 nov 96
Hi Bonnie: If your glaze is crawling mostly near the tops of bowls, it may
just be an application problem. If you glaze both sides of a porcelain piece
at the same time, (especially bowls) the bisque becomes oversaturated in the
thinner top section of the bowl, the glaze takes a long time to dry and does
not adhere well to the bisque. You get crawls! I've had this problem with
some glazes and the remedy is to glaze one side of the bowl at a time,
allowing some drying time before glazing the other side. This may not be the
case in regard to your problem but I thought it might be worth considering
since you are having trouble mainly with bowls.
Craig Martell-Oregon
Tony Hansen on mon 4 nov 96
Hello again:
I saw your note about crawling on clayart.
I'm not sure if you checked the following but if not try it.
http://digitalfire.com/magic/archive/crawling.htm
Cheers.
--
Tony Hansen, IMC - Pulishers of INSIGHT, FORESIGHT, Magic of Fire
134 Upland Dr., Medicine Hat, Alta T1A 3N7 Canada
Phone:403-527-2826 FAX:527-7441 email: thansen@mlc.awinc.com
web: http://digitalfire.com/imc.html
David Hewitt on mon 4 nov 96
>However, on my bowls (the ones that hold about 6-8 cups) the glaze
>crawls a little on the outside near the top rim--sometimes. Also it
>doesn't always crawl on all pots, and even when it does crawl it does
>it in one or two or three places, each no larger than an American
>quarter. I don't remember it crawling on my 1 cup bowls or on my
>plates.
There are a number of possible reasons for crawling, but perhaps the
most common is a small degree of grease or dust on the biscuit surface
before glazing. Handling with greasy hands can be enough. Your problem
sounds as if it is intermittent and so this cause comes to mind.
certainly you want to make sure that this is not the cause before trying
anything else.
Hope this helps.
David Hewitt
David Hewitt Pottery Caerleon, Tel:- 01633 420647
7 Fairfield Road, Caerleon, Newport, South Wales, UK.
URL http://digitalfire.com/magic/hewitt.htm
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