John Boyd on sun 22 jul 01
Dear Potters,
I saw the recent posting by Liz Willoughby and remembered
reading a thread where she commented on another clayarter sounding as though
"bitten by the shino bug." I love that line. I confess to being re-bitten
each day. So, here's a little glaze that I have been playing with. Until I
get back to school (and the bliss of the large, gas fired downdraft kiln) I
won't be able to test in high fire reduction. I wish I had a scanner to
show you the pots, in cone 6 oxidation this is one of the nicer shino-type
glazes I have seen (after collecting and testing almost all of them put out
there). If anyone plays with it, I would love to see scans, I will get you
some when I'm back in school. I also hope some one fires it for carbon
trapping ^10, it has a ^7-10 range.
In the community of potting,
J.C. Boyd
P.S. If anyone (such as Mr.Roy) spots flaws in the glaze, please email me.
I'd like to discuss it. I just got into GlazeChem software yesterday and
I'm amateur, but hooked.
+++++++++++++++++++++++++++++++++++++++++++++++++
+++ shino Saggar Shino (boyd variation) +++
+++++++++++++++++++++++++++++++++++++++++++++++++
Range: 6-10
Firing type: Ox. or Red./Saggar optional
Glaze type: Shino
Color: see notes
Surface: Waxy
Transparency: Semi-opaque
Visual texture: Orange peel/shino bubbles
Flow: None at ^6 Ox.
Flaws: Crazes. Crawls badly if too thick
Testing: Tested at ^6 in Ox.
Date: 07/22/2001
Spodumene 9.24 9.2%
Kentucky OM #4 28.91 28.9% Health warning!
Soda ash 10.55 10.5% Health warning!
Nepheline syenite 51.30 51.3%
-------- ------
100.00 100.0%
Bentonite 2.60 2.6% Health warning!
Variations:
In an oxidation atmosphere this glaze is a translucent white, but accepts
colorants well.
I used an addition of %5 Rutile to achieve an orange/tan/peach variation.
I also threw in %0.5 Redart just for fun, but this added some nice brown
flecks in the glaze for a psuedo-iron spot effect
This was tested on ^10 Laguna B-mix, Plainsman clays ^ 5 P-300 and P-380.
All had similar results with little or no color variation
I also tested this on various Plainsman red clays, with nasty results.
The color is poor, and it will craze very badly.
I am not sure if adding the rutile makes this a true shino anymore.
Notes:
I found the original recipe for this glaze as part of an article in Clay
Times. I have greatly modified the spodumene content from 30.3 grams. I
hope the author will forgive me for forgetting his name in the excited rush
to test this recipe.
If anyone knows of this information please include it and let me know. He
mentioned that this glaze will accept markings in a saggar much in the same
way as a bare pot would.
For my purposes, I fired in an electric kiln to cone 6 oxidation (small
skutt kiln, with dial marked to 10 in increments of 1)
I turn all the switches up at the same time, starting on low with no peep
holes in and the lid propped for 1 hour. I then dropped the lid and plugged
the bottom peep for 30 minutes. After 30 minutes I added a peep plug.
After 30 minutes I would add a peep plug and turn the temperature up 2
notches. I did this until the dials (switches) sat at 5 and all peeps were
shut except for the top two. At this time, I plug the next to last peep
(leaving the top peep open)
and let the kiln soak for 3 (three) hours. After the soak I turn all the
switches to high (still no top peep) until the cone falls in the sitter.
With cone packs the results are slightly different, the sitter will plunge
before the cones fall.
The results of 6 firings over a two month period show that this glaze
needs to sit on the pot a day before firing for the best color. I dip it
once (3 seconds submerged) for the best results. The first couple of uses
(approximately 1 week after screening wet glaze) should be slightly more
orange and transparent than later uses.
I wipe the bottoms of pots almost immeadiately after glazing. If I wait
too long it is a little hard to get off. Visual observations of Soda Ash
migration across the surface of pots during one day in the laboratory (warm
and windy, with the windows open) suggest the high posibilities for an
excellent carbon trapping glaze in reduction (try cone 10, gas).
The bubble texture of shino is something I enjoy and strive for. This
glaze would be really boring in oxidation without the rutile and bubbly
surface. The bubbles also help with good glaze on glaze applications at
cone six oxidation:
Good Glazes over this shino with rutile and redart ^6 Ox. (shino first one
day, decorate the next day) (1 immeadiate dip, or medium brush application)
Glossy Black ^6 Ox. (I believe the one we use comes from ClayArt list)
-Still black, but with titanium dioxide blue/white flecks over bubbles and
edges-
SDSU Texture Crawl (Richard Malmgren's article in Ceramics Monthly or SDSU
ceramics web-Glaze Base)
-Whiter orange color, where thick ther is a noticable crawl/crack pattern-
Other Glazes some may like (same shino and conditions), but I am not crazy
about:
Albany Manganese (I am not sure where the guild got this recipe)
-Reddish Brown to black-
Berry Rust (ClayArt list)
-Almost the same as Albany Manganese but more of a speckly, oxidation
style tenmoku brown
(Incidentally, SDSU over top this mixture in a medium brush application
yeilds and interesting white with brown crackle)
Material Hazards:
Kentucky OM #4 - dust hazard
Soda ash - caustic
Bentonite - dust hazard
Cost: $ 0.82 per kg glaze
$ 0.08 per 100 g batch
===========================
Chemical Analysis
===========================
Na2O 0.75 Al2O3 0.90 SiO2 3.57
K2O 0.12 TiO2 0.02
MgO 0.02 Fe2O3 0.01
CaO 0.03
Li2O 0.09
Alumina:Silica ratio is 1.00 : 3.97
Neutral:Acid ratio is 1.00 : 4.00
Alkali:Neutral:Acid ratio is 1.00 : 0.90 : 3.60
Weight percentages:
Na2O 12.5% Al2O3 24.7% SiO2 57.7%
K2O 2.9% TiO2 0.4%
MgO 0.2% Fe2O3 0.5%
CaO 0.4%
Li2O 0.7%
Volatiles:
CO2 4.7%
Other 4.4%
Loss On Ignition: 8.3%
Molecular percentages:
Na2O 13.6% Al2O3 16.4% SiO2 64.9%
K2O 2.1% TiO2 0.3%
MgO 0.3% Fe2O3 0.2%
CaO 0.5%
Li2O 1.6%
Expansion coefficient: 96.5 x 10e-7 per degree C
Oxides causing abnormal expansion effects: Li2O
Viscosity index: 49.1
Surface tension index: 366.1 (c/6) 362.7 (c/10)
Onset of oxide volatization:
CO,CO2 700 C (c/018) (complete loss)
Na2O 1100 C (c/02) (partial loss)
K2O 1200 C (c/6) (slight loss)
Materials in glaze:
Spodumene supplies SiO2, Al2O3, and Li2O.
Kentucky OM #4 supplies SiO2, Al2O3, TiO2, Fe2O3, and small
amounts of several fluxes.
Soda ash supplies Na2O.
Nepheline syenite supplies SiO2, Al2O3, Na2O, K2O, CaO, MgO,
and Fe2O3.
Bentonite supplies SiO2, Al2O3, Fe2O3, TiO2, and small amounts
of several fluxes; bentonite helps to suspend glazes.
Oxides in glaze:
Na2O is a strong alkaline flux.
K2O is a strong alkaline flux.
MgO is a high-temperature flux.
CaO is a high-temperature flux.
Li2O is a strong alkaline flux.
Al2O3 increases viscosity, prevents crystallization, and adds durability.
SiO2 is the primary glass-former in glazes.
TiO2 adds opacity and encourages crystal formation.
Fe2O3 is a colorant (tan, yellow, brown, red, and black).
In reduction, Fe2O3 -> FeO, which is a flux and colorant (brown, black).
=======================
Cost Summary
=======================
Spodumene 9.2% $ 0.28 per kg glaze
Kentucky OM #4 28.9% $ 0.19
Soda ash 10.5% $ 0.10
Nepheline syenite 51.3% $ 0.23
------ -------
100.0% $ 0.79 per kg glaze
Bentonite 2.6% $ 0.03 per kg glaze
Total cost: $ 0.82 per kg glaze
$ 0.08 per 100 g batch
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