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Ghost Image

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Ghost Image

Postby jennifer » Wed Feb 07, 2018 9:40 am

I have been playing with some ideas from the latest e-book by Paul Tarlow. I made a plate blank using French Vanilla (0137) and Turquoise (0116). I had to fire this piece twice and I wasn't happy with the degree to which the reaction permeated the glass so I cut this blank up into 6 mm thick slices, turned the slices on their sides and put them in other pieces, just as one would with a pattern bar. The first piece had 4 slices in between wider sides of Turquoise (0116) and was fired on Thinfire on a kiln-washed shelf. It came out fairly well. I brushed the old Thinfire off the shelf, put down a new piece of Thinfire and fired another plate blank, this one having 2 patterned slices in between sides of French Vanilla. When I took this second blank out of the kiln I was shocked to see a "ghost image" of the previous piece fused into the shelf side of it. In the 15+ years I've been fusing glass, mainly Bullseye, and frequently French Vanilla as well as Turquoise, I've never had this happen. I always remove old Thinfire and put down a fresh sheet for subsequent firings and I don't remove and replace the kiln-wash each time I do a firing. Can anyone explain why this happened?
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Re: Ghost Image

Postby marykaynitchie » Wed Feb 07, 2018 10:02 am

Wow, that is a really interesting result! I would love to see if we could duplicate it. Can you provide your firing cycle, and explain which parts of the original piece were Turquoise (000116)? I am assuming that the big rectangles of the ghost were Turquoise, but would love to have that confirmed.

Also, what is your shelf made of?
Was the shelf primer fresh, or do you put Thinfire down on fired shelf primer?
What kind of shelf primer do you use?

Looking forward to learning more about this.

Mary Kay
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Re: Ghost Image

Postby jennifer » Wed Feb 07, 2018 2:50 pm

You're right, the large rectangles in the first piece were Turquoise (0116). The first firing schedule was:
200 dph to 1225, 1 hr hold
AFAP to 1490, 40 min hold
AFAP to 900, 2 hr hold
50 dph to 800
100 dph to 700, off.
The second firing was the same except that the hold at process temperature was 35 min. This may seem long but in the first firing the strips of pattern were 9 mm and I wanted to make sure they flattened to the same level as the Turquoise sides which, at 2 layers, were only 6mm thick.

My kiln shelf is mullite and I use Bullseye primer. As I mentioned in my first post, the shelf primer was not fresh, but the sheet of Thinfire on it was. I rarely put fresh primer on my shelves for each firing, but I always use Thinfire, and I've never had any result like this before.

The photo I've attached is of the first piece, top side, which is distorted so I plan to use the shelf side.
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Re: Ghost Image

Postby marykaynitchie » Mon Feb 12, 2018 5:28 pm

Hi Jennifer,

I checked with our Research and Education team. We had not seen this in our studios with Turquoise Opal (0116) but we have seen it with certain other copper colors. The long heatwork required for your project was a contributor to the ghosting.

New info that we plan to add to the notes for Turquoise Opal (0116) in About Our Glass:

During processes that require greater heatwork, such as pattern or flow bar techniques, Turquoise Opalescent has the potential to deposit trace amounts of copper on the surface of the kilnshelf. These deposits may react with sulfur-bearing glasses in subsequent firings. Such deposits may not be visible and can react even when the shelf has been properly scraped and reprimed or, alternatively, when used ThinFire has been removed and new ThinFire is applied. This type of contamination is impermanent and may be burned out/fired out over the course of subsequent firings. A contaminated shelf can be fired with glasses—other than sulfur-bearing glasses—and no reaction will take place. In our studios, we've observed the greatest contamination in subsequent firings with sulfur-bearing French Vanilla Opalescent (000137) and Spring Green Opalescent (000126). For a burnout firing, we recommend a rate of 300°F/hr to 1525°F, with a hold of 1:00 hour.
Mary Kay Nitchie
Bullseye Glass Co.

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Re: Ghost Image

Postby jennifer » Wed Feb 14, 2018 1:49 pm

Many thanks Mary Kay. I've washed and re-primed that shelf and have fired one item on it but since it was Warm White (0920), there was no contamination. However, I will try to burn it off following your schedule. I also bought a new shelf "just in case". Fortunately it's a small shelf and wasn't outrageously expensive. I'm glad to know that Bullseye will add this caution to its glass notes.
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Re: Ghost Image

Postby marykaynitchie » Fri Feb 16, 2018 4:41 pm

Mary Kay Nitchie
Bullseye Glass Co.

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Contamination?

Postby teresa » Mon Aug 13, 2018 10:52 am

Hi! we fire French vanilla (0137-0030-F-Full, 1/19/18, code 10011713, sheet 1103). The piece have ambar transparent stains.

We fire 137 below and tekta above.
145 powder jade green
1426 spring green
1112 aventurine green

1- 222ºC - 663 ª c - :30
2- 333ºC - 810 ºC - :10
3- AFAP - 482ªC - 1:00
4- 83ºC - 371ºC - :00

Mullite shelf without shelfprimer, with thinfire.
Ceramic fiber kiln.
This happened in the first firing.
It could be contamination?

Thanks for your help!!

Teresa Garay
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Re: Ghost Image

Postby marykaynitchie » Tue Aug 21, 2018 11:24 am

Hi teresa,

Yes, those marks on the French Vanilla Opal (000137) are likely reactions to a past firing of a glass with copper in it.

French Vanilla Opal (000137) has a habit of developing unexpected colors and patterns either based on its heat history during the manufacturing process, or based on contact with even small amounts of copper or silver. If you don't enjoy these surprises, we suggest that you use Cream Opal (000420) or Warm White Opal (000920). These two styles are much less reactive than French Vanilla.

If you like surprises, then carry on as usual! ;)

Thanks for posting the photos!

Mary Kay
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