WHC228@AOL.COM on wed 1 may 02
Years ago I had that problem. I noticed that when I didn't bisk my pots I
didn't have that problem. I haven't bisked a pot since.
Bill
Paul Taylor on thu 2 may 02
Dear George
I expect you only get the blisters when you are spraying subsequent coats
and get the pot a bit wet after the under coat has three quarters dried .
What is happening is that the subsequent coat is wetting a very absorbent
under layer that expands un evenly when it gets wet again and if there is
any disturbance of the surface preventing absorption into the bisk - a piece
of dust etc- the glaze lifts away.
The more or less ball clay or bentonite and the finer the grinding the
worse this is. Some glazes will peel off the pot completely others will
crack all over and then crawl like mad in the firing. As Ron has often
posted this is to do with glazes having low drying strength and I have
argued in the past that is the particle size giving greater shrinkage that
causes cracking and blistering. I have come to the conclusion that Ron's
point of view is the better starting point but even so it's the dynamic
between the two that counts - Glaze strength and shrinkage. Things can get
quiet complicated because particle size as well as shape plays a part in
this dynamic. But understanding this can be more a confusion than a help
since there are other variables like the pint weight and the floculation of
the slop to consider.
So what are you going to do . From what you say your glaze is behaving
quite well and has a relatively low shrinkage in relation to its strength
but unlike industry you want a good thick coat of glaze. In the short term
patience should work or ironically impatience. You either have to spray the
top glaze a thin coat at a time waiting for the last coat to dry completely
so the underneath and the biscuit can absorb as much water as possible from
a thin second coat ( seldom works because a slip up - too much glaze and
those little bubbles appear) if you do get the bubbles you can flatten them
with a finger but there is a chance the glaze will crawl and the bubble will
show in the firing as a sort of discoloration in the glaze. conversely if
you can get all the glaze sprayed keeping the whole thing wet ( as if you
dipped it) the underglaze does not get a chance to settle and the whole
glaze shrinks on the pot together. if you are lucky the glaze shrinkage will
not exceed the glazes dried strength and or the fired glaze is fluid with
low surface tension and the glaze will be ok - if not, you will get
crawling.
The other thing to do is to put in some glaze hardener (the wall paper
paste like stuff) in with the glaze . The unfortunate thing is that the gum
will render the glaze impervious as fast as it increases the strength
(hardness) which means you can not get a thick coat; thus you spend a lot
more times spraying loads of thin coats, because the gaze refuses to dry
into the biscuit once you have the first coat on .
This makes spraying a long and tedious process . But you may get lucky
and the find a dilution of the paste less than 0.5% dry wt that gives you
the strength with out the lack of absorption: this depends on the particle
size, distribution and shape of the particles in the glaze . You may get
lucky, but in this there is no measurement too fine. The advantage of the
gum is that you can brush any glaze off that is running and still keep the
coat even.
For special pots that need an even coat and you have the time (It can take
up to eight or nine coats to get a half millimeter ) it is the sure way to
spray to get NO PINHOLES OR CRAWLING - it's the lamentations. Tedious - but
an exact thickness of glaze.
or as I said - you may get lucky and the glaze will still let water in
whilst increasing its dry strength again no measurement to fine - a line
blend 0.5% to 0.05% or less should do it. I would like to know how you get
on if you do the experiment.
I hope this helps
--
Regards from Paul Taylor
http://www.anu.ie/westportpottery
In time the biggest hammer will fail to smash a nut that is the nature of
'disaster' - but also 'hope'.
> From: george koller
> Organization: Infintiy Technology, Inc
> Reply-To: Ceramic Arts Discussion List
> Date: Tue, 30 Apr 2002 20:52:53 -0500
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: Re: Spraying blisters - cause?
>
> "Most often, when I spray, with whatever gun I, I end up getting small
> blisters -about 1/2 the size of a split pea......"
>
>
> Hi Kurt!
>
> I'm not going to theorize on the source but I have a simple and easy
> trick that might just help. I've been wetting the bisque pieces down
> with just a quick once over of water sprayed finely. Seems like it
> might be worth trying just to see if any bubbles form with the water
> spray. I think it makes for a harder surface as the glaze is wetter
longer
> and is allowed to "settle" longer, as when the bisque is dry and thirsty
> the pieces will soak up the first glaze very quickly.
>
> Digging back into my memory of spraying glitches I also remember something
> about my finger prints causing a strange effect. Something more to think
> about?
>
>
>
> Best,
>
> George
Michael Wendt on thu 2 may 02
Many good answers have been given, but try this idea out:
Bisqueware is porous. What is in the pores before we glaze? Air at
atmospheric pressure. Dip a piece of bisque into glaze and it absorbs the
water. So where does the air go? It is forced deeper into the dry portion of
the bisque, and at some point, if enough pressure evolves, it will vent to
the atmosphere in the form of pin holes in the glaze or if the glaze is very
strong and cohesive, various size blisters. I would guess that a good
experiment to try would be to fully saturate a piece of bisqueware and then
let it dry long enough (how long I don't know, maybe a day or two) to reduce
the total amount of moisture near the surface to a level that will accept
glaze coatings well but not so dry that the core is once again filled with
air. I might just try this myself. It could also make the problem worse,
who knows?
For dipping, I wait a few seconds until the glaze is almost dry and no more
pinholes appear and I dip quickly a second time to cover the pinholes. Might
blisters be failed pinholes?
Just a thought,
Regards,
Michael Wendt wendtpot@lewiston.com
Wendt Pottery
2729 Clearwater Avenue
Lewiston, Idaho 83501
1-208-746-3724
wendtpottery.com
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