Tony Ferguson on mon 31 dec 01
Ron,
It has been my experience that certain shinos can rip pots apart--I have
seen it with my own eyes--pot so weak my 1.5 year old could have pulled it
apart with his own hands. Also, I have grabbed the pot off the kiln shelf
as it falls into shards. I have also seen shinos that glazed on the inside
have caused deep cracks on the outer skin of pots, weakening the integrity
of the entire pot--this happens depending on the clay body and the shino and
how it was glazed. Grittner, my old graduate professor, reported this has
happened on his biscuit pots as well--glazing one side or the other but not
both. This has only happened with the outside of the pot where I glazed
the inside and not did not moisturize the outside (stoneware). Porcelain is
another story, different rules. This is an expansion/contraction issue--ask
any single firer and they will tell you about this little joy ride and
lesson learned. It is also a biscuit issue with certain shinos, glaze
application, etc. etc. Not a major problem for most.
Most shinos are not crazed or will not craze--in fact, most shino are in the
3 to 8 % percentage of soda ash (at least the ones I've been collecting for
the last 10 years--from Haystack, Alfred, a variety of Minnesota shinos,
Darmouth, SIUE, other potters, everything I can get my hands on!). Yes, I
admit it, I'm in love with shino. Now, if you are using Davis's shino,
17-19% depending on which version, then you are approaching a much greater
chance of crazing--BUT, this depends mainly on how much feldspar you have in
your body and also grog, or grog like material, or sand. Too little
feldspar, and shino will like to get you a year later or longer with little
flakes falling off. I have found 15% feldspar in a stoneware body to work
for the shinos I use. Porcelain, you don't need to worry about it--there is
enough in it already. If I want crazing, put it on thick and or increase
the grog content. If you like your fingers for breakfast, use pea
grog--puts a whole new spin on crawling.
The type of firing can affect the expansion/contraction in the following
ways:
--depending on the iron content on the clay (at least the body I have been
using)--too much in collaboration with a longer firing can cause serious
crystollobite problems--tension, even exploading pots on a shelf a year
later such as report from Dick Cooter to me when we discussed similiar
problems--feldspar became our friend.
--depending on the amount of body reduction--too much will blacken/carbon
core and set up immense tension--I had pots that from too much reduction
actually cracked within the glaze layer--nothing visible on the glaze--but
twisting your pot you can hear the cracks within the body--also you can tell
by pinging it, striking it with your thumb. I found this to be more of a
reduction/feldspar issue than a purely a shino issue. If it thuds, its a
dud. If it pings, it sings.
Tony Ferguson, Duluth MN Where the mice I keep catching and freeing are
finding refuge, again, in our home.
Something wrong with this explanation but I don't have enough information
> to actually make a difinitive statement about what is wrong.
>
> What I need is the recipe of any shino glaze that is suspected of having
> too low an expansion for the clay it's on - resulting in glaze dunting -
> which this surely is.
>
> Most Shino glazes are crazed - because of all that soda. Crazing (glazes
> under tension) (glaze too small for the clay after cooling contraction.)
> Crazed glazes do not break pots - make em easier to break but not by
> themselves.
> Could be - if a shino glaze had enough lithium and was on a body with
> significant cristobalite - could make a pot break.
> Anyway - to trouble shoot this I need the recipe and a description of the
> clay body and firing cycle.
>
> The other part of the statement below I don't understand - why would the
> type of firing effect the expansion/contraction profile of clay and
glaze -
> don't they all wind up in the same ball park at the end?
> RR
>
>
> >Did you glaze them raw or bisque? I have noticed this before with some
> >bisque pots--shino likes to rip pots apart. If you glaze one side, you
must
> >wet the other--helps with expansion and contraction. If you glazed
> >raw--same expansion problem, just magnified.
>
> Ron Roy
> RR# 4
> 15084 Little Lake Rd..
> Brighton,
> Ontario, Canada
> KOK 1H0
> Residence 613-475-9544
> Studio 613-475-3715
> Fax 613-475-3513
>
>
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Katheleen Nez on tue 1 jan 02
Been using regular strength shino on the interiors of
my large jars (MN Clay NO Light) and 50% strength same
shino Hakame brushed around painted fish with wood ash
from my fireplace (half pinon & half cedar) splashed
all over. No cracking as of yet. (Knock on wood)
Didn't try the twist test...
you could see fotos of my workspace iffen i could
figger out how to load new pix onto my web-site...
(the sound of one head scratchin)
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