Chilly

Picturing the rakes is helping!   thanks so much!!

I have a thought...
If your design structure holds its shape through a bisque firing, what about trying a low fire glaze  instead of cone 6? I have fired many sculptural pieces (which don't need to be fully vitrified) made of cone 6 clay to bisque temperature and finished with various paints and waxes rather than glaze.  I'm wondering if low firing might work for you.

Photos would really help.A kiln sitterr is just a device which, depending on the cone you place between the prongs, will shut off the kiln after a certain amount of heatwork.
Make a log book and note everything you do.Time turned on .position of switch, starting temp temp after 1hr. Temp range, for every hour.
Put a set of cones , one below target temp , one on target temp and one sbove target temp. Place them where you can see them through a spy hole. Towards the end of firing , take out bung to see how the cones are faring.
The cones wil reflect what is happening more accurately that temp. If you ramp up fast it'll take a higher temp to mature the cone, and your glazes, and turn off your kiln. Some glazes won't like that.
If you ramp slowly your cones will drop at a lower temp.
By recording what you do every hour you will see what you need to do.
Photos of kiln and pots overfired will let folk help you more.
Good luck, keep at it!
 

Firing glass on clay has come up a few times on the forum. General consensus of forum members is this isn't a good idea. Several threads on this topic, link below to one of them.
Don't think this would have been why. Reason why the melted glass has craze / crackle patterning is because the glass expands and shrinks at a vastly different rate than a claybody. COE or CTE (coefficient of thermal expansion), same thing, different name.  Sounds like with your original coasters the glass was much thinner than the ones that shattered in your latest firing. This doesn't mean it's safe practice to just put the glass on more thinly, still subject to stresses and the risk of slivers of glass dislodging. 
 

Know that despite how the tone of this might come across, I do sympathize with your ruined project. I mean none of this as shaming, only as information.
You have a relatively wide piece compared to the thickness, no foot rim, a very thin cross section and you glazed only the inside and the top of the rim. You don’t state it, but I’m  going to guess that the glaze layer was a comparable thickness to the cross section of the clay. That alone can be a solid recipe for dunting, even if you haven’t had previous problems with that glaze. That scenario can actually a way to set up a stress test for clay glaze compatibility. 
Add to that chunks of glass of unknown providence, which,  as mentioned above has a VERY different COE than either your glaze or your clay, and fired it far past the maturation point of the glass. I’m not at all surprised these coasters pulled themselves apart with some authority. 
It is, and always will be my opinion that it is ill advised to combine chunks of “found”  glass and ceramic material, ESPECIALLY in functional items. I have stronger words for people who teach this as a beginner project. I know lots of people do it, but it’s still a bad idea. You have unfortunately found a perfect storm example of why.
I do concede in that other thread that there are some circumstances where combining clay and glass do work, or where cullet is included as a glaze ingredient. But to make it work properly requires a very strong technical understanding of the differences between clay, glaze and glass. I will forever disagree strongly with anyone who just melts glass into the bottom of a clay dish. 
If you want some nice coasters to be able to gift for Christmas, even the most technically broken glaze would be a better option than glass. This would be a lovely opportunity to go on glazy, and test out some of those fun, runny zinc glazes that make crystals. I saw today that Birdie Boone and Osa Atoe are working together on just such an example. You could get some really fun effects with none of the headaches of using glass. If you’ve never mixed a glaze before, this is a very simple recipe, and would be a good place to start. Some of the ingredients are a little more expensive, but you probably don’t need large quantities, so it’s still pretty affordable. 
 
 

What they are telling you is true.  I used to melt a lot of glass into the bottom of small pots.  I actually had a small sliver come loose and cut a person.  Not badly, but that stopped me in my tracks.  And @Callie Beller Diesel and @neilestrick are correct about the explosion.  I had a smaller incident with glass in the kiln.  It's simply not a safe practice.  If you have seen commercial coasters with some sort of melted glass in them, I surmise that they were treated with a form of resin to seal the glass.  I have one on my desk.  Which is what prompted me to try it in the first place!  Live and learn.  
Roberta

The glass has a  very different COE (coefficient of expansion- different shrinkage) than the clay and glaze, and it most likely put a ton of stress on the coasters and blew them apart.
To get the piece off the bottom, try sliding a pallet knife/scraper under the chunk. You may lose a little bit of the floor but that won't hurt anything. If it has melted into the floor a bit, put a thick layer of kiln wash on it for the next few firings until it stops melting.

hello, kevin,
i have been single firing for a number of years.  i use the clay and glazes that anyone else would.   i have not found any problems after i started spraying glaze instead of any other method.   i do not even spray the "accepted" way.   i can glaze a kiln load of my work in an afternoon.  it is an L&L kiln about 23x 27 (or so).  i always use the slow glaze that is built into the electronics.   i figure the kiln manufacturer knows more than i do about how to use the kiln to my advantage.
most of what i make is serving ware,  flat, not much higher than an inch or two and not difficult to make.   botanicals pressed into slabs and shaped using wood and foam rubber or pantyhose covered small bowls.   cracker trays and butterdishes.  they sell quickly and are so easy to make that i really do not do the things most throwers  do.  i can throw but at 82 my fingers do not have the same suppleness they did.  an afternoon of throwing might result in waking up to throbbing fingers that night.
the fun in what i make is the choice of botanicals, placement on the shape i use, lots of freeform stuff that dries inside old platters, bowls etc.    you might look at my photo albums .   click on my avatar and choose "my profile" and look for photos from 2016 holiday sale in november.   each of us has an album space available to show our work or an idea to share.  
if you want to try single firing the absolute RULE is   FIRE ONLY TOTALLY DRY PIECES, NOTHING THICK!
 
 

Yes, like Mea said once you remove the magnet on the outside the inside one falls away. If the inside glaze is soft then just drop a scrap of newsprint into the pot first and set the magnet on that.
Another point about why I use a twist tie with the inside magnet is so I can reach the magnet without touching the inside of the pot. Bend it shorter for short forms. For wide open forms like bowls just using the magnet bundle is enough. 
Use enough magnets to get a strong bond between the inside and outside magnet(s), needs to be strong enough to hold together while removing the pot from the glaze bucket (with the suction from glaze) but not so strong that they are difficult to remove.
Rare earth magnets are strong but they are fragile. I've tested ones from different suppliers, I have found there is a difference in the strength of them. They can break but are still useable. I have read that if you have a pacemaker then it's best not to use them. 
 

I’ve been using neodymium magnets as glazing tools too, thanks to @Min suggesting this to me several years ago. Can confirm that this is a game changer! I have my inside magnet wrapped in fabric held on by a rubber band. The fabric helps cushion the magnet so it doesn’t disturb the liner glaze, and the rubber band “stem” is like a handle. My outside magnet has a big eye bolt as a handle. 
I’ve learned to keep them stored on two different shelves, otherwise they will seek each other out and stick together, and it takes a lot of strength to pry them apart! 

Glazing tools and tips brought up in this thread, starting a new thread as this should probably be in the Studio Operations section.
This is how I use magnets when glazing, no tong marks, drips or finger marks on the glaze. It’s been a game changer for me with the pots I dip. Flat bottoms on pots work the best but you can do rounded bottoms, it’s just a bit less stable while dipping. 
- liner glaze the inside like usual
- rare earth magnet(s) wrapped in a scrap of plastic with twist tie inside the pot. twist tie is used as a handle. keep the twist tie below the rim of the pot
- another rare earth magnet(s) with a flat head bolt for a handle on outside of pot
- fishing magnet used for larger pots, they come with a handle so don't need a flat head bolt for a handle
-number of magnets needed determined by pot weight (and base thickness). I try to use just enough to be adequate to pull the pot from the glaze but not more or else they can be fiddly to remove. for tiny pots I use 1/2 pieces of broken magnets.
- hold the pot by the bolt and push the pot into glaze then lift it out. (rotate the pot slightly to have any excess glaze roll around the rim) 
- when the pot is dry enough to touch at base area turn it over and remove magnet(s) from either side. 20 seconds or so for most glazes. I hold the pot on the outside bottom of the pot when flipping over 
- you can slide the pot onto the edge of a counter while removing the outside magnet if you don’t want to wait for the glaze to dry enough to hold onto the pot, or stuff your hand inside the pot to move it to a ware board.
- when I first started glazing like this I got a couple blood blisters, don't pinch your fingers, those magnets are strong!!!! I use the ones from Lee Valley tools. (to separate them use an edge of a counter and push them apart)
Anyhow, that’s it, simple and it works. I tried to think of a way to market the magnet glazing but life got in the way.

Hi np.04,
Is your firing governed by a controller?
If you are able to set cones where they can be watched*, that might be a real help - you could start your cool down when the heat work is completed.
*be sure to wear kiln glasses against the harmful rays and any flying bits!

A cone temperature chart confirms that this is a real effect. A cone number relates to an amount of heat-work and the maximum temperature needed to achieve this depends on how fast the kiln is heating towards the end of the firing.
https://www.overglazes.com/PDF/Orton-Cone-Chart-C.pdf


Did you previously fire by observing the cone drop or just use it as record of the firing?

Yes definitely possible. So silica and alumina do not melt basically on their own below about 1600c/3000f. So they need flux to help melt at a lower temperature The fluxed reaction pretty much takes place in the last 100c of the firing, prior there is just not enough energy to get things going. So temperature and firing speed in the last 100c is super critical, to which PeterH has posted the Orton cone chart above.
As kiln elements age their power decreases and eventually they can’t make a reasonable final firing speed and things begin to over fire. The same goes for anything that slows down the firing such as worn elements AND a cold external firing temperature. In the end though -9c is cold but not that cold so likely the elements are fairly worn. How many firings are on them and to what cone do you routinely fire?
 
 
 
 

Ok I showed up at noon and others where walking thru house buying up stuff. Looks like a small amount of my work had sold . . But the tables where still stuffed.I told the fellow for me its all or nothing deal. I made a quik  rough list and  and down sized my offer to 450 $ cash. I said I have the boxes and paper and will take only my work as there was other ceramics mixed in especailly on the floor under tables (not in the photos)He took the offer which is about 16 mugs for me sales wise these days. I boxed it into 6 Banana boxes  full and dollyed it out feeling great. The womans sister spoke to me and said the lady (her sister) had a stroke and was in full time care now in North Caralina.I said she had the most work of any customer I ever knew  of and gave her my card to thank the sister . I have never met here as 99% of all the work was from a few outlets and they all had the price labels and cards inside of them. Not my own show price labels but those of my Eureka outlets. The largest amout of stuff was from a co-op organic market I pulled out of a year ago as I am slowing it down with a plan. I put a care and feeding tag in every pot  then and all those where still in every pot. Now I have a nearly a 35 cubic feet whole  kiln worth of new old stock. Its insane really. In my 50 years I never in my dreams would expect this to happen. I'm selling the stock twice this time without having to make or glaze or fire it.  Of couse prices have risen since I sold this  originally so I'm really ahead . I'm making 4 dinner plates for the friend who sent me the info last night. She really came thru for me.
There where two tables of pots  like the one photo  as well as a bunch on the floor
Just when you think you have seen or heard it all   -boom something new surfaces .

Finally got round to doing what has been suggested many times before.  Filled a pillow case with dried out clay, thin pieces mainly, and put pillowcase into bucket of water.
Left it for a week (community centre - only go there weekly).  Pulled it out, stood it in a bowl to drain.  Left it for another week.  Poured off excess water.  Left it for another week.
Took it out of pillow case this week, cut lumps off, handed out to class to use.  Perfect and little to no effort on my part - hurrah.
Can't believe how easy this was.  I usually reclaim in a bucket, and removing anything drier than slip has always been a nightmare. 
Started another pillowcase this week from the previous 4 weeks dried out clay.  This has changed my life.
Humidity is low indoors, and heating is on, which has helped with evaporation.  Might work a bit quicker in the summer.

Oh, we have a pugmill, but never enough to fill it.  and you can't empty them!

Oh, we have a pugmill, but never enough to fill it.  and you can't empty them!

Finally got round to doing what has been suggested many times before.  Filled a pillow case with dried out clay, thin pieces mainly, and put pillowcase into bucket of water.
Left it for a week (community centre - only go there weekly).  Pulled it out, stood it in a bowl to drain.  Left it for another week.  Poured off excess water.  Left it for another week.
Took it out of pillow case this week, cut lumps off, handed out to class to use.  Perfect and little to no effort on my part - hurrah.
Can't believe how easy this was.  I usually reclaim in a bucket, and removing anything drier than slip has always been a nightmare. 
Started another pillowcase this week from the previous 4 weeks dried out clay.  This has changed my life.
Humidity is low indoors, and heating is on, which has helped with evaporation.  Might work a bit quicker in the summer.

After my first failed attempt at building my own stands for firing jewelry, I've succeeded in fabricating new stands which held up to ^6 firing! The first time around, I made the stands with ^5 B-Mix and bisque fired them to ^04. They seemed solid enough, but when I fired the assembled stands, the weight of the jewelry caused the stands to sag, which, in turn, caused most of the jewelry pieces to run together and touch its neighbor. The melting glaze joined the pieces permanently. In the cases where the pieces were not co-joined, they were stuck to the stainless steel wire I used to string them because I drilled the mounting holes just big enough for the wire to fit AND I didn't clean the holes of glaze, thus fusing the pieces to the wire.
This time around, I added a strut to the B-Mix w/grog ^5 stands and fired them to ^6. I drilled the jewelry holes bigger and cleaned the holes of glaze. The stands held up to the firing and I had only 2 pieces stick to the wire but was able to break them loose without breaking the pieces. The photos show the first failed attempt, the second successful attempt, the layout of the stands, the assembled stands, and a selection of copper-toned pieces that I thought came out looking pretty darned good! (The penny is to show proportion.) In a number of the pieces, I was going for a raw, organic look with the rough edges which I think will look great as part of a necklace.

Finally got round to doing what has been suggested many times before.  Filled a pillow case with dried out clay, thin pieces mainly, and put pillowcase into bucket of water.
Left it for a week (community centre - only go there weekly).  Pulled it out, stood it in a bowl to drain.  Left it for another week.  Poured off excess water.  Left it for another week.
Took it out of pillow case this week, cut lumps off, handed out to class to use.  Perfect and little to no effort on my part - hurrah.
Can't believe how easy this was.  I usually reclaim in a bucket, and removing anything drier than slip has always been a nightmare. 
Started another pillowcase this week from the previous 4 weeks dried out clay.  This has changed my life.
Humidity is low indoors, and heating is on, which has helped with evaporation.  Might work a bit quicker in the summer.

I think its a all around bad idea. You do not eat off the stone. Why would you put food on that sealer.
You can as Min said have it tested but why bother its not a food sealer. Good money after bad in my mind-let us know what the results are when the lab report comes back.
 
 

Finally got round to doing what has been suggested many times before.  Filled a pillow case with dried out clay, thin pieces mainly, and put pillowcase into bucket of water.
Left it for a week (community centre - only go there weekly).  Pulled it out, stood it in a bowl to drain.  Left it for another week.  Poured off excess water.  Left it for another week.
Took it out of pillow case this week, cut lumps off, handed out to class to use.  Perfect and little to no effort on my part - hurrah.
Can't believe how easy this was.  I usually reclaim in a bucket, and removing anything drier than slip has always been a nightmare. 
Started another pillowcase this week from the previous 4 weeks dried out clay.  This has changed my life.
Humidity is low indoors, and heating is on, which has helped with evaporation.  Might work a bit quicker in the summer.

One of the best ways to reclaim when not having a pug mill!
 
best,
Pres

Finally got round to doing what has been suggested many times before.  Filled a pillow case with dried out clay, thin pieces mainly, and put pillowcase into bucket of water.
Left it for a week (community centre - only go there weekly).  Pulled it out, stood it in a bowl to drain.  Left it for another week.  Poured off excess water.  Left it for another week.
Took it out of pillow case this week, cut lumps off, handed out to class to use.  Perfect and little to no effort on my part - hurrah.
Can't believe how easy this was.  I usually reclaim in a bucket, and removing anything drier than slip has always been a nightmare. 
Started another pillowcase this week from the previous 4 weeks dried out clay.  This has changed my life.
Humidity is low indoors, and heating is on, which has helped with evaporation.  Might work a bit quicker in the summer.