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Silver Stain, PMC and Art Glass
Silver stain is
the black mark left on the ceramic kiln shelf (not
ceramic fiber) when you fire silver (sterling & fine silver) on the shelf.
Even if you don’t see it later it is still present; even after scraping and re-kilnwashing.
The
purpose of this test is multi-fold:
1] To determine which
colors of art glass react with silver so that we can avoid this ugly occurrence
when using art glass with PMC.
2] To educate the public
so they know why they get these marks on the back of their fused pieces from
time to time.
3] To determine if Crystal
Clear (1401) is necessary for use with PMC vs. Regular Clear (1101). Crystal clear is more
expensive and adds a level of complexity in logistics to the glass studio but
not necessarily to the PMC studio. If you use ONLY crystal clear then
there is no problem.
4] To provide data to
Bullseye.
Testing process:
Small squares of art glass were placed on a kiln shelf that contains silver
stain and fired to full fuse. Each square was placed on a silver stain mark.
| Clear 1101 |
No Reaction |
| Crystal Clear 1401 |
No Reaction |
| Deep Royal Blue 1114 |
No Reaction |
| Turquoise 1116 |
No Reaction |
| Aqua 1108 |
No Reaction |
| Erbium Pink 1821 |
No Reaction |
| Neo Lavender 1442 |
No Reaction |
| Spring Green 1426 |
No Reaction |
| Black 0100 |
No Reaction |
| Orange 1125 |
Reacted. Black Mark |
| Yellow 1120 |
Reacted. Gets cloudy, had reddish black mark. |
| Cranberry 1311 |
Reacted. Black mark. |
| Sunset Coral 1305 |
Reacted. Slight discoloration |
| Violet Striker 1234 |
Reacted. Yellowish white mark. |
| Adventurine Green 1112 |
Reacts very badly. Yellowish white mark |
| Medium Amber 1137 |
Reacts very badly. Brown stain w/ yellowish stain in middle. |
| Chartreuse 1126 |
Reacts. Yellowish white stain |
There
are more colors available but these are the ones we
use and sell most often so I started with those. Note that the regular clear
DID NOT react. This has been my belief all along. It may be that WHEN a
reaction occurs the creator thinks it’s because of the regular clear but it, in
fact, is because of some other material present…. for instance…..
one of the metals in the dichroic coating, one of the
other colors of art glass present, perhaps the clear is spectrum or Uroboros ….
Etc…. My own experience with embedding fused glass in PMC has been that clear
1101 does not react.
I will continue to
test all the other colors of art glass I have and will also test with squares of
dichro to see if any of those react. I will also test the new Tekta clear from
Bullseye. I don’t buy Uroboros clear except that I do have some dichro on it so
that will have to suffice as a test. All the art glass is Bullseye. I do have a
few colors of Uroboros and I will test those too. I will test the clear 1101 and
1401 again.
When the glass
does react it discolors the glass in various ways; this discoloration is not
removable even if you grind off the surface. The only efficient way I have found
to deal with this in the studio is to use different shelves for silver and pure
glass. I cut off a notch from the corner of one group because you CAN’T SEE the
stain after you scrap and rewash.
I am sending this
document to Bullseye so that they may be able to tell us which chemical(s) are
responsible for the reaction and may be able to give us a list of all the colors
that contain those chemicals. Perhaps they can also change the formulas so that
we can get non-reacting colors. [SEE BOTTOM OF THIS DOCUMENT FOR MORE TEST
RESULTS.]
Best,
~Nan
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top to bottom, left to
right:
Sunset Coral, Med. Amber
Chartreuse, Yellow, Adv.
Green
Cranberry, Striking Violet
forgot to scan Orange....
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MORE.....
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Testing Phase #2 - squares with PMC3 Paste & wrapped with PMC3 Clay Coils |
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TEST #1: Squares of
colored art glass, clear art glass, crystal clear art glass and dichroic glass
were spotted with PMC3 Paste, dried and fired to 1110 for 30 minutes. 3 Squares
were wrapped with PMC3 Clay Coils - 1 Crystal Clear, 1 Clear and 1 Adv. Green
which is known to discolor very strongly when fired in contact with silver.
RESULTS: The glass
does not change AT ALL at this temperature, i.e. no fusing occurs, no fire
polishing, no nothing... the glass just gets hot.
As
you can see in this image, several colors of art glass and dichro have
discolored already; yellow art glass and pink/teal dichro(upper left corner) are
the most pronounced. The crystal clear and regular clear have not
discolored from the paste OR the coil. The adv green has started to
discolor slightly with both the paste and the coil but it is not so apparent in
the image. The dicho that is discoloring has been spotted with paste on the
dichroic coated side not on the clear side. The same colors of art glass that discolored
from the silver stain are reacting with the PMC.
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TEST #1: Squares of
colored art glass, clear art glass, crystal clear art glass and dichroic glass
were spotted with PMC3 Paste, dried and fired to 1290 for 10 minutes. 3 Squares
were wrapped with PMC3 Clay Coils - 1 Crystal Clear, 1 Clear and 1 Adv. Green
which is known to discolor very strongly when fired in contact with silver.
RESULTS: The edges
of the glass are slightly fire polished at this temperature. As you can see by
the images below, several of the pieces reacted to the silver. The
dichroic coating on several of the pieces also reacted.
Much to my surprise BOTH the clear 1101 AND the
Crystal Clear 1401 reacted slightly at this temp!!!
It's a bit hard to see
in the images but it's more apparent in person; there is a slight yellow tinge
right around the paste. The pieces of clear that were wrapped with clay
also reacted slightly; they don't appear yellow but if you look at the edges you
can see slight yellowing. The green one reacted A LOT!
Left to Right:
Clear w/black streamers, Crystal Clear.
  
Left to Right:
Adv. Green, Crystal Clear, Clear w/black streamers. |
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Testing Phase #1 - silver stain & sterling wire |
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TEST #2: The same
color squares were placed in the kiln, some on silver stain spots, some on
pieces of sterling silver wire. In addition, 5 or more pieces of dichroic glass
were place, coating side down, on sterling wire. These were fired to 1110
degrees and held for 30 minutes. This is the lowest firing schedule for PMC3
listed on the sheet that comes with the PMC3.
RESULTS: The glass
does not change AT ALL at this temperature, i.e. no fusing occurs, no fire
polishing, no nothing... the glass just gets hot. No silver stain occurred
on ANY glass. The Sterling wire discolored slightly and was just beginning to
leave a mark on the shelf but was very faint.
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TEST#3: The same
color squares were placed back in the kiln, some on silver stain spots, some on
pieces of sterling silver wire. These were fired to 1290 degrees and held
for 10 minutes. This is the highest firing schedule for PMC3 listed on the sheet
that comes with the PMC3 (although it can be fired higher).
RESULTS: Again, the
temperature was not high enough to change the glass in any way. No
discoloration to any glass was present. The silver stain marks that were on the
shelf when placed in the kiln are now invisible.
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