Kris Baum on fri 20 mar 98
Am I glad someone else has weighed in on this! I hate to say that my
very favorite glaze (the one I was trying to develop a zillion
different colors of as my standard glaze, because I love it so much)
has a very similar formulation : 50% Gerstley Borate, 20% EPK, 30%
Silica. As it happens, I had already glazed my set of dishes with
this glaze almost a year ago. After everyday use and dishwashing
(and the vinegar test after I read about that one), they look
absolutely fine - nary a scratch.
Ron, I take all your posts to heart with appreciation and grateful
thanks, but my experience with this glaze just doesn't match your
predictions. Is it because you're talking about a longer time frame
than I've experienced - say several years of use? I am working on
some "within the limits" glazes, but I truly hate to give this one
up. (At least, I can still use it on my tiles!)
> ----------------------------Original message----------------------------
> >----------------------------Original message----------------------------
> >>From the following description you might be led to believe this glaze is a
> >good glass at cone 4 to 14 - well it might have enough alumina and silica
> >at cone 4 to 6 but it is also oversupplied with boron - I will predict it
> >is a "soft" glaze and easily scratched - and I doubt it's ability to resist
> >chemical attack. At cone 10 it is seriously short of both alumina and
> >silica and way oversupplied with boron - it's not the fault of the plastic
> >vitrox - a "good" material but rather the abundance of borate.
> >
> >I tried to offer a better glaze in it's place - but as often happens with
> >"simple" glazes, I could not, and keep the characteristics of the original.
>
>
> Dear Ron: figures you wouldn't like my simple glaze. this 50 50 glaze
> appears to be very durable. I don't think you could scratch it with a nail.
> Got it from roger Kerslake and Neely is using it too. God, you're a hard
> man to please.
> Two of my other favs are green- 50 ash 50 river mud and the other is brown-
> 8o Alberta 20 nep sy. I can carry them around in my head. there ain't
> much room left so I keep the glazes simple.
> thanks for keeping my customers alive.
> Cheers,
> tony
>
>
Kris Baum
Shubunkin Pottery
Gaithersburg, MD
USA
mailto:shubunki@erols.com
Ron Roy on sat 21 mar 98
Hi Kris,
I find this interesting - could it be that a glaze that is so far out from
the limits be a durable glaze?
I use two sets of limit formulas and both say this glaze has short comings.
In the first case - with the boron out of flux unity - it has 0.76 boron -
the limit formulas say it should be between 0.3 to 0.5 - that is a big
oversupply.
In the other set - with boron in flux unity - the boron is .43. With these
limits, at cone 6, it should be up to 0.35 - again an over supply. Using
this guide it should have 0.275 up to 0.65 alumina and it only has 0.18.
The silica should be 2.4 up to 4.7 and it only has 1.61.
So you can see why I have drawn the conclusion it is less than durable.
Perhaps what we can do at this point is design some tests to prove what the
real story is. If you are game then send me a message after NCECA (I'll be
back March 31) and we will set it up. The first thing to try is adding some
colour and see if it can be leached.
Part of my intention in reacting to glazes here is - what if somebody
decided to use some toxic material in this glaze and it does leach?
The other problem is the large amount of borate in this glaze - are you
saying this glaze has no problems in the bucket? We have had so many
reports of glazes changing viscosity when large amounts of GB are present.
My other observation is about the low expansion - it is low. You can test
this by applying the glaze to just the inside of a mug. Keep the clay on
the thin side and the glaze a little thicker than normal - no glaze on the
outside. After firing freeze it for 24 hours then - check carefully for
small fractures on edges with a good magnifying glass at all stages. If no
fit problems then freeze again and pour some boiling water in it while
still frozen - be careful because if it is going to crack you don't want to
get burned.
I have worked on similar glazes in my capacity as consultant to Tuckers -
if I can remember correctly - there were dunting problems would be also a
blueish haze when that glaze was contaminated with iron.
I look forward to working with you on this and I would like to hear from
anyone who has any comments on this type of glaze.
Date: Fri, 20 Mar 1998 08:39:41 EST
From: Kris Baum
Subject: Soft Glazes (previously Plastic Vitrox???)
----------------------------Original message----------------------------
Am I glad someone else has weighed in on this! I hate to say that my
very favorite glaze (the one I was trying to develop a zillion
different colors of as my standard glaze, because I love it so much)
has a very similar formulation : 50% Gerstley Borate, 20% EPK, 30%
Silica. As it happens, I had already glazed my set of dishes with
this glaze almost a year ago. After everyday use and dishwashing
(and the vinegar test after I read about that one), they look
absolutely fine - nary a scratch.
Ron, I take all your posts to heart with appreciation and grateful
thanks, but my experience with this glaze just doesn't match your
predictions. Is it because you're talking about a longer time frame
than I've experienced - say several years of use? I am working on
some "within the limits" glazes, but I truly hate to give this one
up. (At least, I can still use it on my tiles!)
Ron Roy
93 Pegasus trail
Scarborough Otario
Canada M1G 3N8
Phone: 416-439-2621
Fax: 416-438-7849
Web page: Home page http://digitalfire.com/education/people/ronroy.htm
rballou@mnsinc.com on sun 22 mar 98
Boron's role as a glass former has been discussed on the list before. To
add to that here's Hamer's (The Potter's Dictionary of Materials and
Techniques) relevant entry on boric oxide:
"BORIC OXIDE. BORON TRIOXIDE. B2O3. A glass-forming oxide which is one of
the most useful of glaze constituents. It melts at a low temperature and is
therefore suitable for low-temperature glazes where it acts both as a
glass-former and a flux............."
"In glaze theory it is considered that there is a molecule of boric oxide
(B2O3) and that this becomes linked in endless chains with itself and
silica (SIO2). To achieve the boric oxide chain one must melt the crystals
of boric oxide. Melting begins at 300C (572F) when the crystals start to
break down. The molecule B2O3 goes through a series of divisions into
sub-oxides with partial melting until full fusion is reached at 700C
(1292F). A useful glass is now formed from the chains of boric oxide and
this does not recrystallize on cooling. It is an amorphous substance and as
such has no melting point but a melting range. Remelting will be in the
range 570c to 600c (1058F to 1112F), which enables boric oxide to be used
as a low-temperature flux (alkali) as well as a glass-former
(acid)........... "
"Boric oxide is a glass-former and therefore works in a similar way to
silica in the glaze. It provides part of the glass network into which are
incorporated the other oxides. One of its greatest merits is its low
melting point and it is often used for this property. Its theoretical
introduction into glazes is simple. It can be substituted for some of the
silica on a molecular basis..... Small amounts (0.1 equivalents) make
appreciable differences in the melting point of the glaze. With matt glazes
it is better to substitue the boric oxide for some of the aluminia. However
the low melting point of boric oxide makes the theoretical consideration of
matts difficult. Once the boric oxide has melted it naturally assists other
actions and therefore has some interaction with the alkalis which would
cause the crystalline matt. "
"BORIC OXIDE AND RECIPE ADJUSTMENTS. Boric oxide is a glass-former (acid)
but also acts as a flux (alkali). Its sesquioxide formula tells us that it
is indeed amphoteric and will act accordingly. Potters often have
difficulty in putting it into its porper place in unity formulae. Its
correct place is with the amphoteric oxides but one must realize that if
the equivalents of boric oxide exceed 0.05 it is difficult to relate the
formula to others which contain none." End of Hamer's entry.
There's more, but I think this is the most relevant part of the entry. The
upshot is that B2O3 plays a complex role in glazes. Including it in the RO
or R2O3 column does not give a full picture of what's happening in the
glaze. It is a glass former, but one with a very low melting point. So to
some extent it looks like we should be adding, mentally at least, all or
part of the the B2O3 to the SiO2 to recognize its glass forming capability
while simultaneously noting its flux-like qualities. Oh yeah, and in the
meantime, don't forget to put it with the amphoterics, too. This sounds a
lot like patting your head and rubbing your stomach; with practice most of
us can get the hang of it. One more thing: boric oxide is used to decrease
the soluability of lead in glazes. Of course, this makes no prediction for
other potentially toxic materials.
As for B2O3 and expansion, Hamer writes about boracitis. Additions of B2O3
to a glaze are useful in preventing crazing up to a point, after which any
more additions have the opposite effect and the glaze crazes. The optimum
amount of B2O3 differs for each glaze. I don't know if this has any
relevancy at all to the expansion characteristics of the glaze in question.
Just add it to the soup pot. Maybe someone else can flesh out this
observation.
It doesn't look like B2O3 will conform to any neat and tidy pigeon holes.
But isn't that what attracts a lot of us to clay? It won't surprise me a
bit if testing proves this glaze to be just fine.
Ruth Ballou
Silver Spring,MD -- where people are starting to feel left out if they
haven't been called to testify before Ken Starr's grand jury yet.
>----------------------------Original message----------------------------
>Hi Kris,
>
>I find this interesting - could it be that a glaze that is so far out from
>the limits be a durable glaze?
>
>I use two sets of limit formulas and both say this glaze has short comings.
>
>In the first case - with the boron out of flux unity - it has 0.76 boron -
>the limit formulas say it should be between 0.3 to 0.5 - that is a big
>oversupply.
>
>In the other set - with boron in flux unity - the boron is .43. With these
>limits, at cone 6, it should be up to 0.35 - again an over supply. Using
>this guide it should have 0.275 up to 0.65 alumina and it only has 0.18.
>The silica should be 2.4 up to 4.7 and it only has 1.61.
>
>So you can see why I have drawn the conclusion it is less than durable.
>
>Perhaps what we can do at this point is design some tests to prove what the
>real story is. If you are game then send me a message after NCECA (I'll be
>back March 31) and we will set it up. The first thing to try is adding some
>colour and see if it can be leached.
>
>Part of my intention in reacting to glazes here is - what if somebody
>decided to use some toxic material in this glaze and it does leach?
>
>The other problem is the large amount of borate in this glaze - are you
>saying this glaze has no problems in the bucket? We have had so many
>reports of glazes changing viscosity when large amounts of GB are present.
>
>My other observation is about the low expansion - it is low. You can test
>this by applying the glaze to just the inside of a mug. Keep the clay on
>the thin side and the glaze a little thicker than normal - no glaze on the
>outside. After firing freeze it for 24 hours then - check carefully for
>small fractures on edges with a good magnifying glass at all stages. If no
>fit problems then freeze again and pour some boiling water in it while
>still frozen - be careful because if it is going to crack you don't want to
>get burned.
>
>I have worked on similar glazes in my capacity as consultant to Tuckers -
>if I can remember correctly - there were dunting problems would be also a
>blueish haze when that glaze was contaminated with iron.
>
>I look forward to working with you on this and I would like to hear from
>anyone who has any comments on this type of glaze.
>
>Ron Roy
>93 Pegasus trail
>Scarborough Otario
>Canada M1G 3N8
>Phone: 416-439-2621
>Fax: 416-438-7849
>Web page: Home page http://digitalfire.com/education/people/ronroy.htm
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