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glaze test for stains and fuming

updated mon 31 may 04


Alisa Clausen on fri 28 may 04

Recently there was a discussion about the possibility of stains fuming a =
tin based glaze.=20

I was particularly interested in this as I had just made an experiment =
based on Ron Roy's theory that Chrome does not need to be mixed into a =
tin based glaze, to fume it. I used MC6G Raspberry glaze, minus the =
Chrome. .I painted the chrome a la carte, on the base of the test tile, =
just under where the glaze line ended. The tile fired to a rosy pink, =
thus confirming Roy's idea that the Chrome will fume the tin base glaze =
from a remote site in the kiln.

Question now is, how remote? =20

Paul gave me the idea to test the stains. Marilu called a major =
(Mason?) US supplier of stains and was told that the stains probably =
would not stain because they are fired during the manufacturing process. =
The probably and should can well be correct from people who know a lot =
about these things. But I am still a test man because I have a cranky =
thirst to challenge the logical. =20

I first tested many stains that would logically contain Chrome as a =
colorant, (am assuming my stains come from Matthey Johnson, but just an =
assumption because most of my other materials do) painted as a stripe =
under MC6G Raspberry, minus the Chrome. I will call this base White =
Cherry for these experiments. All of the tiles fumed. I thought that =
this is inconclusive, because perhaps if one stain fumes, it has far =
reaching strength enough, to effect all of the tiles in the kiln. One =
bad apple and we know the rest. These tiles were fired all next to each =
other on the same shelf. I looked at the very base of the glaze to see =
if the red color was more pronounced on some tiles and others to try to =
determine if one color fumed more than another. There was very =
difference from tile to tile. They were all a pale rosy color.

I made the test tiles again, but fired them as separate as possible in =
my kiln (a big Skutt). It would be obviously expensive and take a long =
time, to fire each tile in a separate firing, to be sure of the =
contaminating effects of one tile to another. I also thought that maybe =
there is Chrome in the bricks of the kiln from previous firings. I have =
only fired very few firings with chrome. This time there were also =
fuming effects on all of the tiles with stains. The strongest pinks =
resulted from stains that were green, black and red. I did a few =
experiments with underglazes. The Amaco Red V382 Velvet underglaze =
fumed very slightly, and my local red underglaze and light pink =
underglaze, did not fume.

These tests were a little more conclusive as I could see a variation in =
the amount of fuming on the tiles by color intensity. I could see that =
the mentioned stains, greens, blacks and reds, fumed more than blues. =20

I can see that stains fume a tin based glaze.

However, what I cannot conclude from these tests, is how much one tile, =
that fumes a lot, can effect an entire kiln load.

regards form Alis in Denmark

May Luk on sat 29 may 04

Dear Alisa;

May I ask how far apart they are? Are they on the same shelf? Have you tried
them on different shelves yet?

You made a very good reminder for me. Recently, I was discussing pink
colouring using rutile and tin in a calcium high alkaline base glaze. I,
together with my glaze group participants, made the assumed [and temporary]
conclusion that rutile may be contaminated with chrome. Now, I can see for
myself in this test.

Well done and thanks for sharing.

London, UK
I made the test tiles again, but fired them as separate as possible in my
kiln (a big Skutt).

Alisa Liskin Clausen on sun 30 may 04

On Sat, 29 May 2004 12:56:19 +0100, May Luk

>Dear Alisa;
>May I ask how far apart they are? Are they on the same shelf? Have you
>them on different shelves yet?

Dear Mary,
I placed the tiles 4 per layer, three layers in the kiln. So to say, the
tiles were about 30 cm. apart, 4 per shelf.

I am reasonably sure after the second results, that the stains tested, all
fumed. Because the tiles with underglazes that were in the same kiln, were
not affected. That would kill my one bad apple theory that one fuming
stain had fumed the entire load in the first firing where the tiles were
all on the same shelf.

Let us know about your Rutile test.
regards from Alisa