Powdered glass questions

[KatyC]

Hello!
I work with clay and would like to incorporate glass into some of my pieces - can anyone give me some advice?

I'm planning on cutting some shapes from a slab of clay and cutting small holes in the shape. I'll then fire and glaze the clay so I've got a glazed shape with holes in. How can I turn these holes into little windows of coloured glass? Is it possible to put powdered glass in the holes and fire with a torch or put back in the kiln?

I know nothing about the eccentricities and behaviour of glass, so would relly appreciate any advice/tips.
Many thanks,
Katy

[shelly]

I would of thought so, but am a beginner so may be wrong.  Not sure if you would trap air in it though which might cause a problem?  I have used enamels on metal with no problem but not onto a glaze, isnt a glaze part glass based anyway?

[Nicknack]

If you have a flat back to your piece (wouldn't work in 3D) you could lay it on a piece of Thinfire paper and fill the gaps with either powdered glass or frit (small pieces of glass - goes from powder to chunks - fine frit should do).  Then fire fairly slowly - 333 C per hour ish - to 800 - 810 C for up to 10 mins, then cool slowly to anneal the glass.  That should give you a basic fuse, but you may get little bubbles in the glass.  I expect someone else will give you better instructions soon.

Nick

[theflyingbedstead]

Hi Katy - welcome to the forum!

I've not tried plique a jour with ceramics, but my initial thoughts would be:
You will need a kiln program to melt and then anneal the glass - this will be different than a normal ceramic firing. I wouldn't even attempt firing with a torch! 
Your glaze will likely have a different COE to the glass you use, so it might be better to not glaze inside the holes!
You will need to use a kiln shelf with shelf primer on it or, better still, use thin fire paper underneath the piece to stop the glass sticking to the shelf and to be as transparent as possible. The bigger the frit size, the more transparent your windows will be. And you might need to fire the piece more than once to get the right thickness for your glass.
It is worth doing test pieces first!

[Nicknack]

There!  I knew someone would come along with better advice!

Nick

[Pat from Canvey]

No advice but your question reminded me of the stained glass biscuits I used to make for the kids when small. In case anyone is tempted. make a biscuit dough and cut out holes for the windows. Before baking, place boiled sweets in the "windows". I can't remember whether I put greaseproof paper on the oven tray. Heres a proper recipe, http://www.bbc.co.uk/food/recipes/stainedglasswindowbi_87505

[Moira HFG]

By amazing coincidence, this is exactly what I'm trying to do at the moment!

My plans have temporarily hit the buffers due to my kiln not reaching a high enough temperature to fire the clay properly (I have more to post on that subject!), but my plan was to lightly glue the shapes to a sheet of thin fire paper with dilute PVA to prevent leakage, then fill the gaps with frit, and fire it like a small mould casting.
If they are very unflat (is that a word?) then I would bed them lightly into a layer of casting plaster. See? I've been thinking about this.

Remember that crushed glass contains quite a lot of air, so will need mounding up a bit to compensate, otherwise the holes will only be half full.

Use a firing schedule for 'full fuse'  (you'll find one on this site), but keep the fusing temperature and hold on the low side, especially if the hole you're filling is less than 6mm thick - otherwise I expect the glass will ball up and form a separate blob in the middle.

I've found a brilliant blog about frit casting; have a look at this -
http://www.morganica.com/bloggery/2012/latest/methods-frit-volume/

No, I don't think you'll get anywhere with a torch.

I look forward to seeing your results! If you show me yours, I'll show you mine..

Moira

[KatyC]

Thanks so much for your replies. I've stocked up on kiln paper and coloured frit and am ready to experiment! I'll come back and let you know how it goes. Wish me luck!

[Nicknack]

Best of luck!  Have fun!

Nick

[KatyC]

So here's the result of my first experiment: http://instagram.com/p/n90rgYRDUF/
I fired bullseye fine frit to 810c with a 10 min soak. It looks like the powder didn't become totally liquid as it's not completely filled the holes (which were packed high with as much powder I could fit in).

Any suggestions on where I go from here? Larger granules? Higher temp (though I'm pleased the glass stayed viscous enough not to leak out of the back of the holes).
Any advice would be much appreciated!

[ajda]

As a first attempt, I think that's pretty impressive! With a little experimenting, I'm sure you'll get there.

How wide are the holes? And how thick has the glass ended up? One of the secrets of melting glass is that it always wants to end up a certain thickness - approx 6mm or 1/4" - unless contained in some way. So a larger amount will spread out until it is all that thickness and a smaller amount will shrink up until it gets there. From the way it's shrunk up against the sides of the holes, it looks to me as if you haven't got enough glass to fill the holes at the magic 6mm thickness. Even though you "filled" the holes with frit, a high proportion of that would have been air trapped between the granules, so perhaps less glass than you thought. You could simply try adding more and re-firing... Larger granules trap less air than fine so that might work (also likely trap fewer bubbles in melting, so the finished window likely to be clearer.)  Alternatively, if you started out with sheet glass rather than frit - cut to circles to roughly fit the holes - you could probably judge the quantity more accurately.

[KatyC]

Ah,that's interesting, thank you-I didn't realise it contracted to its preferred thickness - I assumed it would act like ice and melt to find a level.

I think I will have a go at putting more in and refiring.

What do you think would happen if I fired to 999C? This is the temp I use for my glaze firing, so it would be more economical if I didn't have to do a separate firing for the glass.

[ajda]

I've not fired Bullseye that high myself - but some of the recycled glass I've melted in the kiln seems more prone to devitrification when I've gone up to 1000C or more... but I'm no expert - there might be something else going on... I would simply try it and see - but on a piece that doesn't matter too much! At higher temperatures, the glass may be more fluid and so more likely to flow right through the holes. Could you make a little clay disc to close the underside of the hole completely? It would need to be primed with something suitable so it doesn't stick to the glass and leaves a good surface finish. Based on my limited experience, the best finish would be achieved by priming with boron nitride - very expensive. However, I recently heard about a primer called Primo which might be worth investigating: http://www.youtube.com/watch?v=CWzC4REo6JE

[Pat from Canvey]

With pot melts, the glass is liquid at 930 deg C. I'd put sheet glass into the holes rather than frit. Squares of glass will liqify and fill the holes so you won't need to cut glass in circles. Do you have sheet glass?

[Moira HFG]

I think that looks good for a first try.
Yes, undoubtedly the glass has balled up and clung to one side of the hole. I think the effect is rather interesting!

You can top up the holes and refire to make complete circles - with a second colour of glass if you want.

Next time, you could fire to a lower temp to give a tack fire rather than full fuse, which will give you a less translucent (but still interesting) fill.
Or to give a clear glassy result, either cut circles to size as others have suggested; or use coarse frit which you can mound up over the hole.

And remember to brush off excess glass powder from the front if you don't want little blobs of glass on it!

Sorry I've drifted into a different project, but will post my results soon...