Glass + metal compatibility

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    Sorry if this has already been covered before… couldn’t find any information in my searches.

    I guess this is more of a glass “theory” question but here goes.  If you shouldn’t combine two (or more) different COE glass then why is it possible to use metal inclusions in a fused piece? I mean, the difference between 90, 96, and 104 COE seems such a small deviation when compared to aluminum (248), brass (~210), or copper (176).  Is it because the expansion of the metal happens at a different temperature than that of the glass?  Or is it because metals anneal at a different temperature than that of glass?

    I apologize if this is one of those “stupid” questions.

    Stephen Richard

    The main element in being able to combine metal and glass is size and/or thickness. as the area or thickness of the metal increases you reach a point where you crack the glass.  It would seem to me that the metal is stressed when it is only a fraction of the thickness of the glass, or a small area.  When the metal reaches a critical point – different for each metal – it becomes stronger than the glass and stresses it.

    Stephen Richard

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    So in other words, the smaller the metal inclusion is the less risk of cracked glass.  Makes sense since copper leaf is popular in fused glass.

    Also, since metals are far more maleable than glass, the metals can take a little push and pull from the glass.  Still, there must be some reason (physics, chemistry) why one can mix 90 COE glass with 248 COE metal with fairly stable results but combining 90 COE glass with 96 COE glass is usually a disaster.

    Thank you for your answer, Stephen.  It is greatly appreciated.




    If you were to use the same amount and thickness (not very realistic) of 96, it would be compatible with the 90. The reason that enamel paints are compatible with a wide variety of COEs is that the layer is thin.

    Metal has one other problem that applied glass does not. That is it absorbs and dissapates heat much fast than glass. This can cause thermal shock if you push the envelope too much.


    Barry Kaiser

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