James Dapogny on thu 5 sep 96
Hi out there...
Below is a recipe for a very nice, stable, satiny cone 9-10 reduction
black satin glaze. My question is this: how would you alter it to produce
1) a rich dark brown
2) a softer but interesting cocoa brown.
SATIN BLACK cone 9-10
kona fs 20
custer fs 20
dolomite 15
talc 13
whiting 2
ball clay 10
flint 20
fe ox 3% = 3
co ox 2% = 2
mang 1% = 1
chrome ox 3% = 3
thanks...gail dapogny in ann arbor (still no rain.....)
John Baymore on sat 7 sep 96
----------------------------Original message----------------------------
From: James Dapogny
Subject: glaze alteration
.......Below is a recipe for a very nice, stable, satiny cone 9-10
reduction
black satin glaze. My question is this: how would you alter it to produce
1) a rich dark brown
2) a softer but interesting cocoa brown.
...............................
fe ox 3% = 3
co ox 2% = 2
mang 1% = 1
chrome ox 3% = 3
-----------------------------------------------------------------
No time right now to pop this glaze into INSIGHT and look at it correctly at the
molecular level, but here are a few "from the hip" thoughts:
The black you are currently getting is coming from a heavy saturation of
coloring oxides. Between the four colorants, you have 9% coloring oxides all of
which produce colors in the "dark" tone range. The 2% Cobalt alone would give
you a pretty dark blue. 3% chrome would be a dark (ugly) green. That 9% is a
pretty hefty load of metals to dispurse in the glass matrix, so I'd bet a lot of
it is precipitating out and staining micro-crystals (very small ones on the
surface) of some form of calcium- magnesium silicate that are giving you the
"satiny" surface quality.
I would have some concern with this glaze as it sits for use on food utensils.
Two of the colorants, chrome and manganese, are somewhat toxic. They are
probably involved in staining those micro-crystals. With the saturation of
metals, it is possible that either or both could be candidates for leaching
problems. Only sampling and testing will tell you for sure.
You also have the handling risk to yourself from the raw manganese and chrome
compounds. The former causes a nasty neurological problem or two, and the
latter is a pretty good carcinogen.
Taking this glaze toward the red-browns would involve dropping out the
manganese, cobalt, and chrome, and upping the iron content. Iron reds usually
run in the 7-12% RIO addition to the base glaze. You might want to increase the
alkaline fluxes a tad...... going to more soda spar is a possibility (watch
crazing). Calcium should be subbed for some Magnesium. You might also add a
very little boric oxide (gerstley borate or boron frit) to help favor the red
state of the iron (and reduce crazing tendencies introduced by the upped
Alkalies).
Magnesium as a flux tends to "muddy" up iron colors and make them go darker in
tone, so this glaze as it sits might give you the chocolate type colors that you
want just by using only RIO in the 10-15% range. Darker browns easily develop
from a true oversaturation of iron that percipitates out on the surface (less
invlovement with the silica). Experiment. If you are getting to the point of
"scumming" on the surface you are quite over the useful limit.
To get the iron red, you'll want to get rid of some of the magnesium flux.
Unfortunately, that is one of the things that is probably giving you the nice
"satiny" surface from calcio-magnesio-silicate micro-crystals. Bring the
calcium up. DON'T ADD ZINC as a secondary flux.
Off the top of my head you want a limit something like this for an "iron red"
type base glaze:
KNaO .3
LiO < .1
CaO .5 Al2O3 .3 SiO2 4.0
SrO < .1
MgO < .1
ZnO < .05
Iron oxide in the 7-8% range with just a little manganese dioxide (1-2%) can
tend to give dark browns. Play with this idea on a line blend in the base glaze
as it sits.
There are a couple of ways to get the iron reds. One involves mostly crystals
on the surface, and the other involves crystals suspended more uniformly
throughout the glaze. I'm sure you'll hear more from others on the list.
Iron as a colorant is one of the least toxic in both the raw and fired forms.
However, a serious oversaturation could cause the leaching of iron out of the
fired product. Again, testing is the only true way to tell if the fired product
is "food safe" (whatever the %$#@ that is ).
So a few thoughts to ponder.
.................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086
76506.3102@Compuserve.com
Doug Gray on sun 8 sep 96
Gail
Probably the first thing to do is to cut out all the colorants except
iron. Iron will give you the browns you want and depending on the
amount can give dark to light browns. If you still don't get a light
brown, you could always add some opacifier such as ultrox, zircopax,
etc. Taking the colorants out of the glaze recipe may change the
surface some. Many colorants also act as a flux in the glaze. Iron
is a perfect example. As you increase the amount of iron, you may
notice that the glaze begins to run more than usual. Most saturated
iron glazes, however, have up to ten percent iron, so that gives you
room to experiment. Testing is the key. Good luck.
Doug Gray
Alpine TX
JOHN LINDSEY on fri 26 mar 99
I have a 6-9 glaze recipe that I want to fire to cone 10. any sugestiions
would be appreciated. cornwall stone 21=25 dolomite 21=25 EPK =
14=25
frit 3134 24=25 flint 20=25 zircopax 5=25
| |
|
