John Baymore on mon 26 oct 98
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I have an issue with the article about sawdust firing that I'd like to
bring up here. .............(zap).......... the photos on the front cover
and accompanying the article indicate that the Mayco glaze crackles quite a
bit, and she says in the
article that the smoke gets into the cracks. ..........(snip).........
I think this renders the piece Non Functional again, because of the
crackle and whatever might have been contained in that smoke - she is
using copper sulfate and salt for coloring...- Seems like any water one
put in one of these pieces would seep through the crackle.
(snip)
Great point brought up here. It certainly begs the question about release
of copper... amongst other possible things.
The only way to tell for sure is lab testing for metals release. I wonder
if the pieces have ever even been tested? With the variables in this type
of firing..... even if one piece tests out OK the next one might not.
You'd have to do tests on a very large block of works to see if the overall
pattern showed that there was no concern... so you could extrapolate that
they really WERE safe to market as =22functional=22.
I'd be leery of selling this type of work as =22functional=22 without having=
a
block of good documentation as to the safety. I'd also want a good product
liability carrier =3Cg=3E. To suggest this technique to novice potters as
suitable for functional ware (without hard testing data to back it up) is
questionable, at best. One wonders if they have it?
Another issue in sawdust fired ware is the purity of the sawdust burned.
If you get it at a lumber yard...... what is in it? There are many
products cut at a lumber yard that contain a lot more than pure wood. The
most dramatic case if pressure treated lumber ..... contains nasty stuff.
I would wonder how sawdust fired pieces that used some or all PT sawdust
would test out for arsenic release? I don't know ...... but I'd sure check
before using or selling them. (I wouldn't breathe the smoke either =
=3Cg=3E.)
Add this one to my other PMI related concern about there being very little
mention of potential hazards of refractory ceramic fiber in the same issue
(see my recent post on fiber use).
Another one of concern to me in this same issue is the volumetric glazing
article. Volumetric measures allow HUGE variations in the batch to batch
composition of a glaze. To promote this manner of glaze origination for a
base to which colorants are added for functional ware (plate with
commercial red stain pictured in the article) is quite questionable.....
particularly with a large (soft) boron content in the base glaze suggested.
Try this at home, folks: Scoop out a level cup measure of gerstley borate
and weigh it on a gram scale. Dump it out. Scoop up another cupfull....
but pack it down slightly before leveling and weighing. Weigh it. Repeat
this using various scooping methods (against the side of the bag, from the
tightly packed stuff at the bottom of the bag, from the stirred up fluffy
part, scoop it up with a treaspoon and fill the cup measure, and so on.
Vary the method you use to scrape off the cup level. Even try to do it
exactly the same each time. Weight varies doesn't it?
Here is my weight test variation (in grams) that I just went out to the
glaze lab and ran on a level cup of gerstley borate. One thru eight I
tried to do basically the same each time.
=231 198.4
=232 195.2
=233 196.9
=234 186.6
=235 185.4
=236 193.3
=237 182.3
=238 190.1
=239 212.5 (packed down)
=2310 174.5 (sifted into cup with spoon)
=2311 201.4 (scraped against side of bag)
So..... a range of 174.5 to 212.5 for a cup of gerstley borate. That is a
spread of 38 grams or 17.9=25 variation (based on the highest weight). 38
grams out of roughly 190...give or take. A significant error factor.
How exactly does she scoop it =3Cg=3E?
These weight variations will result in variation in the fired glaze
composition. If one batch leach tests out OK with the inclusion of 1 unit
(which can vary in weight too) of a commercial stain containing oh
.....say..... manganese, copper, cadmium, or whatever......., will the next
be OK too?
On the soapbox..........
For god's sake.... a new triple beam balance acurate to one tenth of a gram
is only around a hundred bucks. If you can't invest this small amount in
the necessary tool to do reasonable glaze formulation then maybe you
shouldn't be doing glaze formulation on your own yet.
We are messing around with a big chemistry set here =3Cg=3E. We routinely =
buy
and handle chemicals that the average person is never exposed to in the
powdered form and which our doctors are usually AMAZED that we can buy and
handle, if we ever tell them. (Lithium Carbonate in the purer medical grade
is a controlled psychoactive drug.... for crying out loud.)
The traditional =22folk potters=22 that worked imperically with things like
volumetric measure for glazes didn't have the access to chemically pure,
finely ground, air-floated materials like we do, nor did they have the
range of more exotic (toxic) things. We can buy most any chemical and dump
them all together, stick then in some heat, and HOPE that we have created
something that is as stable as possible. Then without testing them
chemically, we give them to family and friends or sell them to customers.
There is a lot to this glaze making thing. If you make functional ware you
need to understand a bit of it. Take a glaze calc course. It is part of
the necessary stuff to learn....right up there with throwing and
handbuilding and firing.
Food for thought.
Off the soapbox..........
Carrying this furthur I got the (single try) weight for a cup of each of
the mateials listed in the base glaze and ran it in the weight variation
through a basic analysis in Insight. I am assuming the use of Gerstley
Borate for Colemanite on a one to one substitution. It comes out as
follows:
Sumi's Volumetric Clear (PMI)
GERSTLEY BORATE 595.2
MAGNESIUM CARBONATE 116.4
WHITING 236.1
F-4 FELDSPAR 1325.5
FLINT 754.2
CaO .54
MgO .13
K2O .07
Na2O .25
Al2O3 .27
B2O3 .48
SiO2 2.83
(BTW....colemanite is almost unavailable in the US right now. Does the
author mean Gerstley Borate where she says colemanite? Colemanite and GB
are different materials...and have different chemical formulas.)
The analysis shows that the B2O3 is almost at the upper limit, and the SiO2
is near the lower limit. With all that boron....this is a soft glaze. So
varaitions in weighing....... excuse me....scooping =3Cwg=3E........... =
could
have a significant impact on this glass as far as durability goes. (If
colemanite is used for the Gerstley the CaO goes to .64, the Na2O drops to
..15, and the B2O3 goes to .46.)
Volumetric mixing of glazes has its place. I use it often. But the
selection of specific materials to use, the context it is used in, and the
way it is presented is important. An educator has to think about these
things in the selection of material. To present it in this way to novice
ceramists is not very good, I think.
Best,
.......................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
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