Gavin Stairs on fri 10 sep 99
At 12:17 PM 9/9/99 -0400, Llewellyn Kouba wrote:
...
> Some time ago we had a thread about putting Liquid Gesso in a glaze to
>be used for refiring. Can anyone be a bit more specific as to the
>approximate amount of gesso needed? Say I have about two cups of glaze for
>my spray gun- how much gesso would I add? Can you overdo it etc. Any
>input,. applications etc. would be appreciated.
Llewellyn,
Liquid gesso is just whiting (and nowadays often titania) in a painting
medium. The medium is usually acrylic emulsion. The ones containing
titania are a lot more expensive.
Note that whiting is a glaze flux, which may change the character of your
glaze. It may make it runny or matte.
Titania, on the other hand, is an opaquing agent. So clears may end up
pastel shades, etc.
The part of the gesso that will help you in applying glaze over fired glaze
is the medium. For water based glaze slurries, the acrylic medium will
disperse well, and will act as a binder, adhesive and wetting agent.
Acrylic medium can be bought by itself. If what you want is the
adhesive/wetting agent, I'd suggest you try that instead of gesso. I
expect that a little will go a long way. It is doing about the same thing
as CMC gum, for example. If the glazed surface is dirty, you can try
adding a little of some kind of detergent or soap. Sodium laurel sulphate
is a relatively non-foamy kind often used in baby shampoo. Try a bit of
baby shampoo. Or photo lens cleaner. Or photo-flo. Or clean the pot with
an ammonia cleaner, like Windex, or houshold ammonia.
As to how much, I'd suggest taking a small, measured cup of glaze, and
adding medium and/or baby shampoo a few drops at a time, until the glaze
coats and sticks to the surface you are trying to reglaze. You will
probably want your glaze to be stiffer than normal, if you are doing a true
glaze coat. If you are doing on-glaze decoration with strongly coloured
enamels or glazes, you will need thinner glaze and more medium or gum. If
the medium is higher than maybe 5-10%, you are beginning to have a paint,
whith a strong binder film when it dries. This binder film can pull the
glaze off the pot during firing, or bubble and spatter, resulting in
crawling and spreading defects. So less is more.
CMC or some other gum is doing about the same job (also a suspending
agent), but it forms a much weaker paint film. So the spalling and
spattering problems will be less. You might be better of with some pottery
suspension agent, rather than acrylic medium.
Gesso is used in painting as an under coat which provides the basic white
ground for the painting. Very important for transparent colours like
acrylics and thin coats like egg temperas. So it is a true paint, which
forms a cohesive film of binder with filler included for colour. A
previous thread on this list discussed the problems of firing acrylic
paints: sometimes unpredictable effects, you have to fire off a lot of
binder, spalling, remnant muddiness, only the mineral colours (like cobalt
blue, the iron (earth) colours) leave any reasonable colours, colouring is
usually weaker than the paint, etc, etc. The same would be true of
underpainting with pure gesso before overpainting with glaze or enamel, If
that is what you are trying to do, then I suggest a very thin layer with as
little binder as possible, and, for a white (though probably not
predictable) underlayer, as much titania as possible.
Hope this helps. Gavin
It's been a long time since I did any painting. I did some texture
painting with gesso, but that was many years ago. I still have a big,
dried up jug of gesso, and some knives, etc. One of my paintings is in a
place of honour in my step sister's flat in Paris. In the w.c. So she says.
Nina Jones on fri 10 sep 99
------------------
Gavin, can this also be used to apply glazes (low and high fire) to unglazed
vitrified porcelain?
Nina D. Jones
Southside Chicago
=40 njones=40winston.com
=3E=3E=3E Gavin Stairs =3Cstairs=40stairs.on.ca=3E 09/10/99 11:08AM =
=3E=3E=3E
----------------------------Original message----------------------------
At 12:17 PM 9/9/99 -0400, Llewellyn Kouba wrote:
...
=3E Some time ago we had a thread about putting Liquid Gesso in a glaze =
to
=3Ebe used for refiring. Can anyone be a bit more specific as to the
=3Eapproximate amount of gesso needed? Say I have about two cups of glaze =
for
=3Emy spray gun- how much gesso would I add? Can you overdo it etc. Any
=3Einput,. applications etc. would be appreciated.
Llewellyn,
Liquid gesso is just whiting (and nowadays often titania) in a painting
medium. The medium is usually acrylic emulsion. The ones containing
titania are a lot more expensive.
Note that whiting is a glaze flux, which may change the character of your
glaze. It may make it runny or matte.
Titania, on the other hand, is an opaquing agent. So clears may end up
pastel shades, etc.
The part of the gesso that will help you in applying glaze over fired glaze
is the medium. For water based glaze slurries, the acrylic medium will
disperse well, and will act as a binder, adhesive and wetting agent.
Acrylic medium can be bought by itself. If what you want is the
adhesive/wetting agent, I'd suggest you try that instead of gesso. I
expect that a little will go a long way. It is doing about the same thing
as CMC gum, for example. If the glazed surface is dirty, you can try
adding a little of some kind of detergent or soap. Sodium laurel sulphate
is a relatively non-foamy kind often used in baby shampoo. Try a bit of
baby shampoo. Or photo lens cleaner. Or photo-flo. Or clean the pot with
an ammonia cleaner, like Windex, or houshold ammonia.
As to how much, I'd suggest taking a small, measured cup of glaze, and
adding medium and/or baby shampoo a few drops at a time, until the glaze
coats and sticks to the surface you are trying to reglaze. You will
probably want your glaze to be stiffer than normal, if you are doing a true
glaze coat. If you are doing on-glaze decoration with strongly coloured
enamels or glazes, you will need thinner glaze and more medium or gum. If
the medium is higher than maybe 5-10=25, you are beginning to have a paint,
whith a strong binder film when it dries. This binder film can pull the
glaze off the pot during firing, or bubble and spatter, resulting in
crawling and spreading defects. So less is more.
CMC or some other gum is doing about the same job (also a suspending
agent), but it forms a much weaker paint film. So the spalling and
spattering problems will be less. You might be better of with some pottery
suspension agent, rather than acrylic medium.
Gesso is used in painting as an under coat which provides the basic white
ground for the painting. Very important for transparent colours like
acrylics and thin coats like egg temperas. So it is a true paint, which
forms a cohesive film of binder with filler included for colour. A
previous thread on this list discussed the problems of firing acrylic
paints: sometimes unpredictable effects, you have to fire off a lot of
binder, spalling, remnant muddiness, only the mineral colours (like cobalt
blue, the iron (earth) colours) leave any reasonable colours, colouring is
usually weaker than the paint, etc, etc. The same would be true of
underpainting with pure gesso before overpainting with glaze or enamel, If
that is what you are trying to do, then I suggest a very thin layer with as
little binder as possible, and, for a white (though probably not
predictable) underlayer, as much titania as possible.
Hope this helps. Gavin
It's been a long time since I did any painting. I did some texture
painting with gesso, but that was many years ago. I still have a big,
dried up jug of gesso, and some knives, etc. One of my paintings is in a
place of honour in my step sister's flat in Paris. In the w.c. So she =
says.
Gavin Stairs on sun 12 sep 99
At 03:32 PM 9/10/99 -0400, Nina wrote:
>Gavin, can this also be used to apply glazes (low and high fire) to unglazed
>vitrified porcelain?
Well, doggone, Nina, now I'm gonna have to rush right out and try it and
see. But if it works on glaze, it should work on porcelain, too. The
problem with all organic binders is that they burn out just before the
sintering starts, so there is a critical time when the binder is gone, but
the glaze is not yet sticky. It is at this point that the crawling may
occur, especially if the glaze is shrinking (lots of carbonates, Mg?). So
I wouldn't say that organic binders will answer all problems. I will say
that the whole (well, maybe not the whole) powder metallurgy industry is
based on the use of organic binders, and there they burn out of the whole
body of the piece long before the metal begins to sinter. You don't want
to give the pieces a bumpy ride through the tunnel kiln! The best
technique is to use just enough binder so that when it dries, it draws the
solids in the glaze into contact with eachother and the pot. Then, when
the binder goes away, the particles have been intriduced to eachother, so
to speak, and may retain some cohesion. But, if there's too much binder,
the particles remain separated by the organic film, and no cohesion between
particles is developed. So when the binder leaves, the glaze may crawl.
The story is similar with a dirty (read greasy, oily, from aerosols in the
air, or from too much handling) pot. The oily film prevents the particles
in the glaze from getting close enough to the pot to form a tentative
adhesion. So, even if you succeed in getting the glaze to apply wet, you
may have problems later on. So, for example, you may see finger marks that
weren't apparent earlier. A surfactant, like a detergent, helps to
disperse the oily film, and brings the particles into closer proximity.
But again, less is more.
My feeling is that in glazing, these are perhaps useful techniques for
dealing with occasional problems. It is probably better to simply work
with clean pots bisqued to the right maturity. And in applying glaze to a
slick surface, the first thing might be to wash the pots carefully in
detergent, using rubber gloves, and don't touch them before glazing. Or
work with the pots fresh from the bisque, before they get dirty. Water
(and water based glaze slurries) will wet a clean glass surface. If it
needs a bit of help, a mild surfactant, like Photo-Flo, available from a
photography shop, may be all that is required. If that's not enough, then
a little bentonite, or gum, or acrylic medium. There are also wetting
agent used by glass painters and cell artists that might work. What works
best for you depends on your glaze and technique. Try anything. Glue,
mucilage, gelatin, soap, starch, soup, spit... All in moderation.
Gavin
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