help- slab work cracking when I melt glass on it

[sarah staub]

Hey all, 

using white cone 6 stoneware- B-mix 5 WC-401 I have been making some little 8-inch coral wall hangings-  I throw bowled plates and fill it in with coral looking bits. They dry slowly and they usually make it through the glaze firing fine but then I like to add some glass (I bought some stained glass and mosaic glass pebbles- photos attached (the ones I bought are different colors but same type)) and put them in for another glaze firing and then they often crack at this step. I try not to use too much glass since I know that can cause cracking  and often they just crack a little bit in the back but sometimes it goes all the way through (photos attached since Im not good at describing this) I have 2 questions: 

1- after they're bisqued, can I put the glaze on them, let it dry and then put the glass on top so that I just do one glaze firing or is there any cost/benefit to doing it separately? - I did it separately this time because I decided I didnt like how they looked just with the glaze.

2- I've heard you can put sand underneath pieces to be like a shrink slab but I fire in a community kiln and they dont seem to be open to that; would making little clay coils or rolling out cylindrical strips and putting them under the corals when I do the glass melting help to prevent cracking?  -I figured they were cracking because of friction getting caught on the kiln shelf 

any other thoughts/suggestions on how to do these?  I just made a few more and trimmed the back of the bowled plates to add a little foot thinking that might help since there will be less surface area touching the kiln shelf but they are still drying 

thanks!

[CactusPots]

Trippy.

Porcelain will stick to the shelves, so I'd use alumina in wax as a foot coating.  Nobody can object, because it looks just like wax.

I fire pieces with bits of glass in a wood ash mix, right in the glaze fire.  I can't see any reason to twice high fire these.  Porcelain is nasty enough, in my opinion. 

[sarah staub]

thanks! I dont use porcelain- I use white stoneware fired to cone 6- but does this all still apply?

thanks

[liambesaw]

Glass and clay have very different coefficients of expansion, it's possible that the glass has even broken the clay.

[Bill Kielb]

Looks like it cracked from friction although not many have success with glass especially long term as Liam said. Double glaze firing definitely not good But making sure it doesn’t stick during firing is really important. Fire on a cookie coated with alumina or grog so it doesn’t catch as it shrinks and grows. 

[Callie Beller Diesel]

So the difference in COE’s between clay and glaze are pretty big as Liam and Bill mentioned. But other factors that ceramic folks that like to add glass to things are totally unaware of is that a) glass also melts at much lower temperatures than we typically work with. Fusing happens at cone 022 at the top end outside of blown glass, and b) glass typically needs to go carefully through more phase changes than just quartz inversion, and crash cooled through the rest of the cycle in ways that stress ceramic materials into dunting. 

Glass when combined with ceramic doesn’t just fail because of the differences in COE. If you’re using typical ceramic temperatures and cooling cycles, you’re boiling it, denaturing most colours, and altering the structure of the glass, totally removing any temper it might have had. Glass needs to be annealed properly to have strength. But since we’re not dealing with functional anything, there’s some leeway to be had here.
 

If the cookie trick doesn’t work, the next thing you might try is to investigate proper glass slumping firing cycles. Google search terms “warm glass” should find you some good references. I would fire and  glaze your piece as normal, and add the glass in a third, very low fire fusing or slumping firing that goes through an annealing cycle. A glass slumping firing is easier to do in a kiln that’s designed for it, but you can do it on a pottery kiln that you can program several ramp ups and cooling holds into. Pottery kilns are designed to get hotter than glass kilns, and tend to go through the important glass zones too quickly without the ability to program them in this way. It won’t work in a cone sitter model unless you have a LOT of glass experience, or want to hover over it with a pyrometer. 
 

edited to add: if you think glazes can be technical, they’re nothing compared to glass. 

[Chilly]

As above, but I'd like to really REALLY emphasise that clay and glass were not designed to go together.  Read through this forum and you will find hundreds of posts where people are having difficulties to get a good match between clay and glaze, which are designed to go together.

Glass can explode away from the clay.  It might not happen today or tomorrow, but might be in 6 months time.

You wouldn't put petrol in a diesel-engined car.  Don't mix clay and glass.

[CactusPots]

I like to get the wood ash drips normally associated with wood fire.  I've been putting a fine glass product designed to be used for reflective signs in the wood ash mix.  I get nice drips without any issues that I've noticed.  I've also been using a small chip of glass on the shoulder of a handled pot to get drips.  Most of the time I'm using a shino as a base.  That's a really stable glaze that probably helps.

[neilestrick]

Your pieces have a lot of variation in thickness, so refiring can be difficult. You'll need to go slowly, and getting the pieces up off the shelves on coils would help. However you can just put the glass in when you glaze fire and avoid it altogether. Or better yet, find a good glossy glaze that fits your clay body and puddle that in there instead.

[PSC]

I don't think the glass is breaking your ware. Looking at those crack i can see glass melted into the cracks. If it was the glass cracking the pieces it would happen as it cooled not still liquid. I think its either the uneven clay thickness or some other stress that has happen either while drying, bisquing or the final firing.

After your bisque tap the item with your fingernail...if it has a microcrack before it goes into the glaze  kiln the tap will sound flatter than if there is no crack.

[LinR]

The group that I work with have found that B mix 5 really doesn't like to be fired to cone 6.  Lin

[Roberta12]

10 hours ago, neilestrick said:

Your pieces have a lot of variation in thickness, so refiring can be difficult. You'll need to go slowly, and getting the pieces up off the shelves on coils would help. However you can just put the glass in when you glaze fire and avoid it altogether. Or better yet, find a good glossy glaze that fits your clay body and puddle that in there instead.

A glaze like Water blue would give you some of the same effect.  Your pieces are lovely and obviously you have spent a lot of time on them.  Since they are decorative and not functional, you don't have to worry about the glass slivering off and cutting someone (that's why I stopped mixing glass and clay years ago) but it can put undue stress on already complicated work.  Alumina in the wax on the bottom, along with putting the pieces on a cookie, and maybe exploring some lovely blue glazes to give the same effect?? After all, isn't glaze just glass?? sort of??  Just some thoughts.  I love your hanging system, btw!  Nice!

Roberta

[Magnolia Mud Research]

1 hour ago, Roberta12 said:

After all, isn't glaze just glass?

Yes!!!!!  and thick glass must be cooled slowly to reduce internal stresses.  The “composition”of most commercial glasses  have significantly different expansion coefficient than the ceramic material; that’s why cracks show up.  Glazes have the same problem; potters call the cracks fit problems.  
LT

[Babs]

Yeh get same effect by using glaze in varying thicknesses and being mindful when firing.The variety of thicknesses needs slow and careful up and down firing.

Wax and alumina  or fired on sausages of clay 

[Sorcery]

Seems the one cracked because the glaze came thru the hole and stuck to the shelf?

If so, seems they didn't crack for the same reason, which changes the investigation.

Unless your hang holes are severely turned out, I don't see them catching. And that piece shouldn't have much  trouble shrinking over a clean shelf.

I would make one plain round hole, easier to hang for end user, and less likely to catch. 

One hole more whole!

And way skip the glass, there must be good blues to use!

Sorce

[Sorcery]

2 minutes ago, Sorcery said:

changes the investigation.

 

How are the holes cut?

Reckon it's all....in the hole.

 

Sorce

[dhPotter]

To get the light coloring effect try Watercolor blue or Watercolor green. Use a thin application.

[sarah staub]

Thanks so much everyone!!! I was using a hole making tool (hollow cylinder with angled sharp edge) and then carving out another little bit on top but I might try just doing one hole like Sorcery said I had tried a few different glazes and pooling the glaze in places but didnt get the same depth/effect that the glass gave but I think Ill keep experimenting with glazes and maybe add some underglaze to bits before I put the glaze on to add a little more color and maybe just adding a few fine pieces of glass in places and Ill make some cookies for them to sit on too. Someone else had recommended wax and alumina so I think Ill buy some of that too and just keep experimenting! itll be awhile before Im back in the studio again but ill post some photo updates once I have them!

thanks again!!

Sarah

[Stephen]

On 5/11/2020 at 9:09 PM, sarah staub said:

e but then I like to add some glass (I bought some stained glass and mosaic glass pebbles- photos attached (the ones I bought are different colors but same type)) and put them in for another glaze firing and then they often crack at this step.

nah, glaze is not glass and glass should never be used with pottery, functional or decorative. They each have different properties and they will not work together, ever. It's not a question of if they will separate but when and when it does there's likely a safety issue of some sort to deal with for the unlucky person dealing with the failure. Might be serious and really not worth the risk. I got this from a high level employee of one of the worlds largest art glass companies and they constantly tried to get people to take down instructions for mixing the two. Their product info on their website clearly warned artist not to do this. It's not an opinion it's a fact and it really frustrated them that often people wouldn't listen. Lots of cool glaze looks that are not dangerous to anyone.   

[Stephen]

On 5/12/2020 at 3:12 AM, Chilly said:

As above, but I'd like to really REALLY emphasise that clay and glass were not designed to go together.  Read through this forum and you will find hundreds of posts where people are having difficulties to get a good match between clay and glaze, which are designed to go together.

Glass can explode away from the clay.  It might not happen today or tomorrow, but might be in 6 months time.

You wouldn't put petrol in a diesel-engined car.  Don't mix clay and glass.

ha ha, I thought I remembered your post from years ago, just surfed and saw it and you really put it well both here and this one from 2013. 

Glass can and will explode off the clay, as they have incompatible COE.  (Put simply, they expand and contract at different rates/ratios.)  They're a ticking time-bomb.  Protect yourself from explosions and litigation.  Just don't do it.

I think the vast majority of artist just don't know this and its a shame since it can be so dangerous down the road. I was told that often times on a piece that appears to work and is not handled much or moved around it might be years or maybe when its moved from one environment to another ie; dry room to wet or cold area to hot area. If it just cracks or crazes that one thing but if it falls off when handled or all over something like a food prep counter etc. 

anyway sure people read this and re-think it.  

 

[liambesaw]

It's funny. I was at my dad's house earlier and saw some coasters I made in college using broken glass on the surface, it has big chunks missing now, but it's been over 20 years, I can't imagine where those missing pieces ended up, hopefully not in someone's hand

Edit: this was fairly thick glass though, maybe an eighth of an inch or so, nothing like the OPs pieces

[Chukchi]

The glass is trying to spread out because it's melting and at the same time the clay is shrinking.

It could be that you're just using a little too much glass.

[Rae Reich]

If you like the look of small bits of different colors, as in the photo of the unfired piece, you could color various "pebbles" with underglaze colors or wedge some Mason stain into the bmix.

Nice work!