Brad Sondahl on tue 29 jul 03
I've noticed with my crystalline glazes, which don't work when too thin,
that apparently as the batch of glaze is used, the viscosity goes down.
To compensate, I often take out up to a quart of water towards the end
of the bucket.
I'm thinking this may be why some crystalline glazers claim their glazes
don't store well.
I don't mix my glazes in the scientific manner some folks have
mentioned, of weighing a known volume of glaze on the triple beam to get
its exact density, but I'm thinking if you are one of those persons,
perhaps you could compare a batch of glaze at beginning and end of
bucket to see if this is a real effect. I don't think it would matter
what the composition of the glaze is, although some ingredients may be
more prone to it than others.
In theory, it doesn't make a lot of sense. Glaze is deposited on pots
as water gets sucked into the dry bisque (or greenware, if one firing),
causing the glaze to form a dry layer on the surface. If anything,
this system might result in glaze getting thicker, from top to bottom of
the bucket.
Also, since glazes settle quickly, unless one is constantly stirring,
the glaze might thicken towards the bottom of the bucket.
Another variable would be how vitrified the bisque is, which would
affect how much water absorbs into the clay, and how thick the layer of
glaze would be. I feel confident that my bisques are fairly standard.
So, has anyone else experienced this, or better yet, made measurements
as to the changes?
Brad Sondahl
--
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Avril Farley on wed 30 jul 03
----- Original Message -----
From: "Brad Sondahl"
To:
Sent: Tuesday, July 29, 2003 3:47 PM
Subject: Glaze density changes, as bucket empties
I am not entirely scientific in my approach, but do always measure all the
glaze ingredients and use a standard amount of water per dry kilo.
When I have a spare minute I make up 3 - 4 kilo dry weight glaze base
batches at a time to which I add - at the time of making - 3% CMC. For each
dry kilo I add one litre of water, then whiz with my magic (ex) kitchen
soup blender stick. I keep this base in a lidded plastic bucket until I am
ready to add oxides. At this point I will rewhiz the whole base batch for a
minute or two, then decant approximately 350 millilitres of base per
required colour variation to which I add the oxides in about 20 millilitres
of water. This is seived through an 80 mesh into plastic screwtopped picnic
beakers which are very effective in stopping natural evaporation. I rarely
use more than a quarter of the base glaze at a time, and have found no loose
water on the top of the batch in the bucket throughout the storage period
(maximum 4 months to date). The remaining decanted glaze mixes also remain
stable (in fact I am still using small batches of oxided glaze that I made
up over two years ago with no noticable lack of crystal formation), and
which need only a cursory swipe with a paint brush before use. My glaze is
brush applied to biscuit Limoge porcelain fired to 1040 c with 30 min soak.
The walls of my pieces are about 3mm thick and are glazed both sides.
I found 1000c biscuit too porous for my recipe, but this may be in the
nature of the clay and my throwing thickness rather than as a standard.
I think this consistency must be the result of CMC suspension plus no loss
of water through evaporation.
As an aside, I love your website and laughed my socks off at the mug
instructions.
Avril in the Forest UK
avril@soudley44.freeserve.co.uk
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