John Hesselberth on fri 19 feb 99
I have continued to fill in a few missing references in the literature
search I have been doing on the history of limit formulas and what they
might tell us about glaze durability. For those interested, I have
posted an update to the literature search article at:
http://www.frogpondpottery.com/glazestab.html
I have discovered one thing, though, that is different from "conventional
wisdom" of the messages here on Clayart. Namely, neither Cooper/Royle
nor Green recommend that boron be included in unity when doing glaze
calculations. Cooper/Royle, I now believe, are just pointing out that
boron has fluxing characteristics, but the examples they give all show
boron OUT of unity. They do list boron with their list of fluxes,
however, so an initial reading of this book might easily give the
impression that they have included boron in unity. Ron Roy and I have
been carrying on an off-list discussion of this and, after rereading
these references, he agrees that boron does not belong in unity when
using the Cooper/Royle/Green limits.
The specific piece of evidence that led me to this finding was to note
that both Cooper/Royle and Green give the same limit numbers; however
Cooper/Royle give them only for stoneware while Green extends them down
to earthenware. In the earthenware range, Green shows boron limits as
high as 2.0! Clearly boron cannot be in unity, in the derivation of
these limits, with a limit above 1.0. Sooooo, please be aware that BORON
SHOULD NOT BE INCLUDED IN UNITY when using ANY of the published limit
formulas including Cooper/Royle/Green.
Interestingly, both Ron and I think it may make sense, on occasion, to
include boron in unity--for example, when you are trying to substitute
one flux for another. However, getting a grip on this will require many
more months of experimentation and thinking. I am sure Ron will have a
follow-up message on this subject to give his views.
I am also trying to reach Emanuel Cooper to learn more about the criteria
on which his limit numbers were derived (e.g. hard data on things like
hardness, metal leaching, and crack resistance OR "good glass---most were
derived on the "good glass" criteria)--so far I have not received a
response.
Two other notes on the revision of my article on glaze stability: 1) I
have now included a very interested reference by Eppler thanks to a lead
from Pat Pawlicki of the PEMCO Corp. Eppler has developed a "figure of
merit" to predict whether or not lead will leach from a glaze and 2) I
have added a couple paragraphs on computer glaze calculation programs
which I will develop further as I get time.
More later,
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is time for potters to claim their proper field. Pottery in its pure
form relies neither on sculptural additions nor on pictorial decorations.
but on the counterpoint of form, design, colour, texture and the quality
of the material, all directed to a function." Michael Cardew in "Pioneer
Pottery"
| |
|
