EJR Beads

Author Topic: Baking on glass items like jars and vases  (Read 4106 times)

Valvaren

  • Jr. Member
  • **
  • Posts: 50
Baking on glass items like jars and vases
« on: January 04, 2014, 01:33:29 AM »
Just wondering if you guys have any tips on how to go about baking polymer clay covered glass. Foolishly I didn't look it up before I tried a piece tonight, thankfully the clay was okay but the glass underneath was warp and broke (it was already cracked but I think it would have cracked anyway) I read this is due to the sudden temp changes. Is there anything I can do to prevent this in further pieces?




monkey ann

  • Illuminati
  • Sr. Member
  • *
  • Posts: 347
Re: Baking on glass items like jars and vases
« Reply #1 on: January 04, 2014, 10:05:34 AM »
I guess you could try putting the clay covered glass item in a cold oven and let it warm up gradually as the ovens gets up to temp.  I do this if I am baking food in a ceramic or glass dish.

Have used glass jam jars as bangle formers, no problems with these going into a hot oven, glass is quite thick though.
Ann from Croydon, Surrey

ejralph

  • Administrator
  • Hero Member
  • *****
  • Posts: 6029
  • They're not people, they're Hippies
    • EJR Beads
Re: Baking on glass items like jars and vases
« Reply #2 on: January 04, 2014, 10:16:50 AM »
First off, I have to say I love your project - it is wonderful!

As for your technical question. If the glass has warped in anyway, that indicates it isn't glass. Or if it is glass, it might be covered with some type of plastic coating. Because glass itself will not warp at the temperatures we bake clay at. Glass could break and crack with thermal expansion, but it won't actually warp or distort shape.

So before covering anything in clay, you need to be sure it is actually glass and not a glass/acrylic combination or just very good imitation glass of some type.

Secondly, as Ann says - you can put the glass into a cold oven and slowly bring it up to temperature. Once the clay has baked, shut the oven off and allow the piece to cool slowly back down to room temp before removing it from the oven.
This will minimise any risk of thermal shock to the glass and also allow everything to contract at a slower rate.

Another thing you can do, if you're getting cracking or warping in the CLAY layer is to leave a little expansion gap somewhere, so the clay has room to "breathe" around the glass. This can be as simple as a small gap left at the seam where the clay wraps around the glass.

Emma
Emma from Sunny Sussex, UK
www.ejrbeads.co.uk Projects, articles, bead galleries & more
www.ejrbeads.co.uk/shop Art beads, clay & jewellery supplies

Valvaren

  • Jr. Member
  • **
  • Posts: 50
Re: Baking on glass items like jars and vases
« Reply #3 on: January 04, 2014, 06:10:18 PM »
Thank you Emma!

Perhaps you are correct with it not being actual glass. I'm not sure if it did actually warp I thought it did because I could no long put the nozzle part back on the top but that could have just been due to the cracking.

I'll try what you both suggested with letting it slowly heat up and cool, should I adjust the baking time then since the clay will be sitting in the heat for longer? Like say if I would back it for twenty should I adjust so its only at full temp for like 15 or 10?

I'll try leaving the seam as well. The thing I used for a base was just a cheapish vinegar type container so perhaps it wasn't actually glass, plus it was already cracked which is why I just had it laying around to try.

I picked up a little glass mason type jar to try a second one so i'll do what you guys suggest and let you know how it turns out :D! Now if I can just conquer magnets and pins i'll be getting somewhere.

Karolina.S

  • Hero Member
  • *****
  • Posts: 1595
Re: Baking on glass items like jars and vases
« Reply #4 on: January 04, 2014, 07:17:35 PM »
To me in the top photo it looks a lot like plastic, the way it is cracked and also becuase of the foggy surface, but so little is visable so i could be wrong.
Timrċ in the north of Sweden. Always with my polymer clay eyes open.
http://www.flickr.com/photos/77424684@N04/ http://www.facebook.com/Hobbyrian

Sylvia

  • Sr. Member
  • ****
  • Posts: 310
Re: Baking on glass items like jars and vases
« Reply #5 on: January 04, 2014, 08:36:38 PM »
. . . and be careful about magnets.  Everything I have read, and a couple of teachers too, says that baking magnets causes them to lose significant strength.  What you do is you leave a magnet-sized cavity on your piece, bake it and glue the magnet in after.
Chalfont St Giles, Bucks, UK

Carrie

  • Hero Member
  • *****
  • Posts: 4259
    • Beads from the Coast
Re: Baking on glass items like jars and vases
« Reply #6 on: January 04, 2014, 10:02:45 PM »
You can also carve out a magnet-sized hole afterwards.

Great project! 

Ceni85

  • Newbie
  • *
  • Posts: 2
Re: Baking on glass items like jars and vases
« Reply #7 on: March 25, 2016, 05:11:17 PM »
My problem with this is that the clay cracks and not the glass. Don't know what to do

MilleD

  • Hero Member
  • *****
  • Posts: 969
Re: Baking on glass items like jars and vases
« Reply #8 on: March 25, 2016, 06:01:18 PM »
Hi, you may be better starting another thread as this one is so old, it may get more responses.

But if the clay is cracking - are you conditioning it correctly/enough?  Are you baking at the correct temp for the brand you are using?

Or could it be you are putting clay over something that changes shape and size when baked such as wood?

Alan

  • Full Member
  • ***
  • Posts: 166
Re: Baking on glass items like jars and vases
« Reply #9 on: April 27, 2016, 03:52:05 PM »
At the temperatures clay is baked at, there should be no problems with glass, but as others have said, it would be best to put the piece in a cold oven and let it heat up with the oven temperature.  My oven doesn't take long to heat up - so I would just bake the clay for the usual length of time.

Pyrex glass is especially made for putting in ovens.  Because of the way glass expands during heating - one side expands quicker than the other, and that's why it cracks - especially if going from room temperature into a hot oven.  So pyrex glass could go in a hot oven, but normal glass should be heated up slowly and, as Emma said, allowed to cool down with the oven too - or at least handled very carefully if removed when hot as the slightest tap could crack it.  If glass begins cracked, the heat will exploit that weakness and it will crack further.

My problem with this is that the clay cracks and not the glass. Don't know what to do

Clay probably expands at a different rate to glass - so if the glass inside expands more than the clay and there is nowhere for it to go, it will crack the clay.  Try doing what Emma suggests and leave a gap at the seam so the clay can move with the glass as the glass expands during heating.  Again, heating slowly will mean expansion will be more gradual and the risk of cracking will be reduced. 
Alan from wet and windy, but deceptively mild Cumbria, UK.
https://etsy.com/uk/shop/AlanCordiner

ejralph

  • Administrator
  • Hero Member
  • *****
  • Posts: 6029
  • They're not people, they're Hippies
    • EJR Beads
Re: Baking on glass items like jars and vases
« Reply #10 on: April 28, 2016, 08:54:51 AM »
I would try dropping the temperature a little - cracked clay often indicates too hot a baking temp and it could be the glass is holding or increasing the heat, or the clay (most likely being a thin-ish layer) is baking quicker than usual.

So I would heat up and cool down slowly and drop the temp a little and see if that helps.

Emma
Emma from Sunny Sussex, UK
www.ejrbeads.co.uk Projects, articles, bead galleries & more
www.ejrbeads.co.uk/shop Art beads, clay & jewellery supplies