Chipping off of a TS slip or just the fading of a smoke fired pot!

[nadesprockett]

Hi all-
I've just started smoke firing some coil pots I've made and love the finish I have achieved...
But has anyone ever had chips of the pot break off ??!!

This was an experimental beginning....
I made a large sculptural pot using a raku clay for strength, and made up a TS slip to put on over burnishing and then burnish again...( the TS was applied many times and completely covered the grog in the raku clay)

I made the TS from powdered red earthenware clay ( only thing the suppliers had....)
Bisqued to 900'c in an electric kiln....Smoke fired using newspaper and kindling......took about maybe 30 mins- so just a light smoking...

All great and fine....
I noticed then, maybe 3 weeks later that what looked like chips ( ie small pieces) of the actual surface of the pot was coming away! I can see the bisqued raw raku clay where the TS seems to have literally fallen off the pot surface!

Is there any way I can stop this from happening?!
Is this because I used an earthenware red clay to make the TS and its on a raku stoneware body?
Do I need to fire/ smoke it for much longer to make it gel better?


And this leads to another problem!!!
If you smoke fire a burnished coiled pot, with a TS applied ( again using either red or white earthenware clay powder to make the TS)
Can the smoke marks eventually fade?

I got a shock when I saw that a buff stoneware clay, coiled pot thats been burnished, ( no TS) the carbon black/ greys/ taupe colours achieved in the smaoke firing ( again about 20 - 30 mins with newspaper)have faded dramatically over 3 months...

Is there any way to stop this from happening?

Surely, you could never sell a pot that has the potential to fade?
Thats a money back guarantee from an angry customer!

Has anyone heard of making up a TS slip with porcelain powder ( so it is stoneware firing...)

As you can see, I'm floundering here....
Any advice, pointers , own experiences , other knowledge would be greatly appreciated!

I have a chance to put a piece into an exhibition soon, but if theres fading and chips flying......oh no!

Thanks again
Nadesprockett

[powdermillpottery]

I believe your problem is in your TS or the application. First thought that comes to my mind is that it is too thick. Second thought is if you are going to apply TS, then don't burnish the pot as you really don't need to. I'm going to give you a TS recipe below that was given to me by Charlie Riggs. I have had great luck with this TS. It is as follows:

1) Crack open a cold beer.

2) Dump in 3 1/2 gallons of water in a bucket.

3) Stir in a tablespoon of sodium silicate and/or soda ash.

4) Mix in 17 lbs of XX sagger OR 9 lbs. OM 4 ball clay and put the
bucket up on a table.

5) Finish your cold one and wait a day (24 hours).

6) Siphon off the top 1 gallon.** (This includes the little bit of
water off the top) This top 1 gallon is your terra sig. You can use
immediately or store it for several years.

Dump the rest. It's just dirt!

That's it. Dip it, spray it, brush it.

I usually do 3 coats and polish with a piece of panty hose in between coats and after the final coat. Hope this helps.

Jeff Gieringer
Powdermill Pottery

[Andy Clift]

Hi-

I am presently having trouble getting the Clayart server to
my mesages, so I'm just sending this directly. If you want to
forward it to the list from your own account, feel free.


Date: Fri, 11 Jul 2008 12:53:11 -0700
From: Andy Clift <aclift@CLAYSTATION.COM>
Subject:


This was an experimental beginning....
I made a large sculptural pot using a raku clay for strength, and made
up a TS slip to put on over burnishing and then burnish again...( the
TS was applied many times and completely covered the grog in the raku
clay)


Here's a problem. If you burnish before applying terra sig, it will
be so smooth that the terra sig won't stick well as it dries. Remember
that the terra sig has a different shrinkage than the clay underneath,
even if it's made from the same clay, because terra sig only includes
the finest particles. Using a different clay makes the possibility of
cracking off even worse, applying it very thickly also makes it worse,
and putting it over a burnished surface - well, that's three reasons all
working together to make your terra sigillata pop off:

1: underlying clay is burnished
2: underlying clay is very low-shrinkage compared with the terra sig
3: terra sig was applied very thickly.

Using a groggy body is tempting, but it is really the root cause of
all three conditions. If you must use a groggy body, find one with
very fine grog, so you won't need to burnish and/or apply thickly in
order to get coverage, and so that the shrinkage rates won't be
so discrepant. There's a reason why most terra-sig-covered work
is small, and it's partly because fine-grained clays which work best
with terra sig aren't good for really large work.


Is this because I used an earthenware red clay to make the TS and its
on a raku stoneware body?
Do I need to fire/ smoke it for much longer to make it gel better?


It doesn't matter what temperature of clay you make the terra sig
out of, because it won't ever get fired hot enough to vitrify. If it did,
the burnish would be destroyed. Firing longer won't help any of the
problems mentioned above, either; it'll mostly make them worse.
What makes the terra sig stick is partly the sintering of the particles
during firing, but also the mechanical bond of all those small
particles locking into the texture of the coarser clay below. That's
the part that you are preventing by pre-burnishing the clay. The
sintering (semi-melting) of the terra sig just can't compensate by
itself. If you fire hotter, you will destroy the burnished surface look
which is the main reason for using terra sig in the first place, AND
you will cause it to shrink even more, exacerbating the tendency
to flake off the clay underneath.


And this leads to another problem!!!
If you smoke fire a burnished coiled pot, with a TS applied ( again
using either red or white earthenware clay powder to make the TS)
Can the smoke marks eventually fade?


Not the marks make of actual carbon. You probably also got smudge
marks from stuff that scorched, and the scorched materials stuck to
the clay as the smoke and hot air carried the particles around the
firing. Smoke often contains more materials than just carbon, but
most of those materials aren't stable, and will fade. If it faded, then
it wasn't carbon.


Has anyone heard of making up a TS slip with porcelain powder ( so it
is stoneware firing...)


Again, the firing temperature of the clay you make the terra sig out of
has almost no effect on the end result! If you fire it above about ^010
or so, you will lose the burnish, regardless of the clay it's made out
of. Porcelain is not a good clay to make terra sig out of anyway; its
particles are fairly large, so most of them will go to waste and only
a tiny amount can become terra sig. The clays most people prefer
for terra sig are chosen mainly for particle size, to get the best yield
from the process. Color may affect your decision, but the firing
temperature of the source clay is irrelevant. You can use stoneware
for the object, and make your terra sig out of stoneware clay (or
porcelain, or whatever), but you still can't fire it above low
earthenware temperatures without undoing nearly all the burnished
look that you are seeking.

-Snail Scott

[nadesprockett]

Thanks a lot Aclift- that clears up a lot of my wonderings!
Didn't know about the not burnishing before the TS application...that makes a lot of sense...
Regards
Nadesprockett