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I have been slumping float glass of a 50/50 plaster/silica mould. i have been getting a discolouration in the mould that is leaving a small yellow mark in various places on the float glass. i have used distilled water for the mix and changed my plaster supply several times. i am now thinking that it may be unmissed silica but I am not sure? any suggestions?

any chance that there can be some form of silver in your silica? silver foil, silver filings, stirring with a silver spoon? rosanna

Peasey wrote:I have been slumping float glass of a 50/50 plaster/silica mould. i have been getting a discolouration in the mould that is leaving a small yellow mark in various places on the float glass. i have used distilled water for the mix and changed my plaster supply several times. i am now thinking that it may be unmissed silica but I am not sure? any suggestions?

Steve

Sometimes my alumina, plaster, diatomeacous earth, mix gets a yellow cast to it, but it has never marked the glass yellow.

I have the same question as Rosanna, is there any silver or cadmium kicking around?

What temperature are you working at? How many firings do you get from a mold?

thanks Rosanna and Bert for your reply. I have been careful to seperate the glass and silver , so I can pretty well guarratee that there is no silver contamination. I am firing up to 800 and I can get anything from one to three firing out of each mould. The discolouration isn't obvoius in the plaster until the mould is fired and then the marks corispond with the discolouration in the plaster, a range of seperated little dots ?

Yes, but has silver EVER been in contact with the plaster during a firing? Silver will contaminate even though you can't see it.

Tony

Tony Smith wrote:Yes, but has silver EVER been in contact with the plaster during a firing? Silver will contaminate even though you can't see it.

Tony

That's it. If there has ever been silver around it is the culprit. When glass stainers use silver stain it is done on a bed of whiting (calcium carbonate). The whiting is entirely discarded. If some gets on the shelf, it will continue to mark glass.

Just to add a bit to the fray. I don't use silver--in any form. Yet, my fiber has a yellow cast on it from firings, and I have wiped yellow off my fired glass. I personally think a potential culprit is black glass, since I have had more evidence of it after firing black. The answer to this would be most interesting.

Thanks one and all for your replies. I will follow up the silver contamination and be more careful with the working relationship of the two materials but I don't think their has been any cross contamination. i rang up my supplier just in case other people had the same problem. She informed me that the silica i was using container small traces of iron and suggested I try a low iron based silica. So I have ordered new silica and plaster , tumbling the mix and using with distiled water. i will keep you informed of the results but if anyong has any other suggestion I would love to hear them. Thanks heaps for the help.

Barbara Cashman wrote:Just to add a bit to the fray. I don't use silver--in any form. Yet, my fiber has a yellow cast on it from firings, and I have wiped yellow off my fired glass. I personally think a potential culprit is black glass, since I have had more evidence of it after firing black. The answer to this would be most interesting.

Barbara

My dry powder mix gets a yellow cast to it, but it does not mark my float glass. I think that the stuff you are seeing is cadmium and the key is that it wipes off. Sometimes when I wipe newly enameled glass, the paper towel rubs off the yellow powder. It doesn't sem to effect the color that is left on the glass.