Chilly

Trapped air doesn't cause explosions. What causes explosions is moisture in the clay. It can take a long time for all the moisture to leave the clay so oftentimes people leave a vent hole, not for the moisture to escape but for the air circulation to allow moisture to facilitate moisture escape. Air trapped inside work can rarely cause clay cracking, but not exploding,  as the air expands when heated.
I would do whatever you need to do to get that wood  skewer out of there. As the clay continues to shrink it can crack as the skewer won't shrink. 
Welcome to the forum 

That's number one on my wishlist for next vehicle.  In a country where we rarely get temperatures at or below 0c.

is that space big enough for me?   temp here went down to one, 1, degree F a few days ago.

@ChillyI thought the steering wheel heating was maybe overkill, but I like it more than I thought I would.

My way of wording the same info that @Callie Beller Diesel and @Roberta12 have said is:
We have enough problems getting clay and glaze that are sold as compatible, to be compatible, without adding an ingredient that is scientifically not compatible.
You wouldn't dream of putting diesel fuel into a petrol-engined car (or vice-versa), so why would you want to mix clay and glass. 
Petrol and diesel both come from crude oil, but are not compatible.  Glaze and glass may both contain silica, but they are also incompatible.

+1 for the minivan, as much as it makes 20 year old me want to yell at my current self!
All of the reasons Mea mentions above are the same ones for me. Comfortable drive, easier to park in cities, and you can stuff that sucker with a LOT of pots. 
Last year we traded in our ‘07 Honda Odyssey for a ‘16 Chrysler Pacifica.  The Odyssey’s automatic doors were going, and it didn’t owe me anything. The new one was mostly chosen for its availability, but I’m very much in love with the factory remote start that also turns on the heated seats and steering wheel. It’s got a few inches more of cargo space than the Honda, and I’m surprised how much that affects how efficiently you can pack it. I can get way more in. 

Ok if you read my post on buying back over $3500 new unused pottery from a horders estate sale for $450 then you know how starnge and rare that might be.
well lighning strikes again on that front
Today the day after Christmas at 4 pm I was bookkeeping on the desktop on a very rainy windy day. The phone rings and its the estate sale guy from 3 weeks ago. They found three more large boxes of my pottery and they need to get it gone. He says $100 for it all and you have to pick it up today. I had sanitized/reorganized my van on the only dry day Xmas afterr stuffing it full from taking xmas booth down xmas eve. I Jumped in and drove the 35 minutes back to that house. Last time it was jambed full-this time thay has a box moving trck and all that was left in the whole house was my pottery that they just found in 3 boxed in a hidden space in large kitchen under a counter back water area.  My guess is 1-2 k worth of stuff.I'll go thru it in a few days. Just like last time all new all still with sales tags on bottoms. I found out that this person bought my pots wrapped them and boxed them and put them away like xmas ornaments -all 5 k worth. She may have been my best customer-well its a toss up as another from the east coast who runs a private jet rental service may have her beat ? Hard to say as she bought dinnerware sets and lots of st=uff over time where as this lady bought it all in 3 years from one organic market before I quit them.
When I left the house a hour ago it was 100% empty of all contents-I drove home in disbelieve as Santa Extended xmas one more day at least to me
Merry xmas and Happy Holidays from Linda and Mark

I would say in the UK 1300c and cone 10 and are pretty much interchangeable even if most kilns wont manage 150c/h for the last 40 min of the firing up to 1300c.
 
 
I would test what voltage you have in the studio over a few days and get the elements made for that voltage. The UK is supposedly 230v but I get 245V at my house so any elements designed on 230v will draw more power on 245v. Would be worse the other way around running 240v elements on 230v as then you will be down on power.
Just realised it is a front loader which tend to be more complicated than top loaders. Just a guess at the elements inside being 4 in the side walls and two in the floor. The side wall elements may all be the same spec but the floor will possibly be different.
Need to know the length of the tails (probably double what they are now so you have extra to play with/cut off, maybe 9-12") what length each bit of element is in the wall and how long each hairpin is. Also need to know what gauge the wire is, the internal diameter of the coil (maybe 1/2 or 3/4") and how they elements are wired up. Once you know how they are wired up we could possibly figure out what resistance they should be.
A bad drawing to maybe help.

As someone who’s been firing an analog kiln outdoors in the Canadian winter for the last few years, I can confirm that -9 is not enough to make a difference in your firing speed. I don’t start getting much slow down until about -25 or so, and even then, it’s only about a 15 minute difference.
Since your elements are likely worn, you can still continue, but definitely include the cone packs this time, and make sure you’re checking the firing every 15 minutes for the last 2 hours or so. You want to be able to turn the kiln off yourself once the cones fall.  If you fire overnight to avoid peak electricity rates, maybe start it about 2-3 hours later than you would normally, so you’re awake for the end. 

Thank you Min, Callie for your suggestion! I'm cautiously optimistic as the 2nd try worked! Here's the picture. Fired with pin setters beneath the 2 dips plus the sides!

I’m not a regular potter in the sense of day to day occupation. Summertime, weekends, and holidays are bursts of work. I’ve had to learn to pace myself better physically. If I’m doing it right the work helps keep me in shape and things are fine. If not, something will hurt and take a long time to resolve. Digging my own clay really brought it into focus. I can carry ten pound loads to my truck all day or try and carry two or three fifty pound loads and pay for it later. Doing too much in an hour leads to being able to do nothing for two weeks, the opposite of productive. So, accepting a sustainable pace has probably been the biggest thing for me. 
I have to do regular physical exercise. As much as I wish it, clay alone doesn’t keep me fit. Even in the summer when I’m doing a lot, it’s not all heavy lifting and wedging. I exercise and that makes the hard work easier. 
Also, I do everything I can think of to make clay less physically taxing. Use softer clay, arrange the space for economy of movement, throw standing, get tables the right height, things like that. Thinner kiln shelves. I’ll get that right next time I order some, haha! 
Speaking of videos, another favorite of mine is this one of Michael Cardew. Plopping down ten pounds of clay on the wheel, he says, with a laugh, “What a merciful lovely life it is that you, at the age of eighty one, you can still do it! I say, you get weak, but improve in cunning. You get to know where to not waste your energies.” Absolutely charming. Hope I can still jam like that when I’m eighty one. 
 

gosh, i just read all these posts and mine again.   realized that what i said did not include this line  "i do find flaws, maybe one in two or three firings"  those are the ones i was talking about.  the original post sounded like they happen every day.  not so.

In my outlets-meaning gallery gift shops and organic markets its always 100% perfect. In my shows its 99.9 perfect .S cracks are toast. Any flaws are extremely minimal and saved for later.
I have kept the seconds for my xmas booth these days and let a customer who buys a piece (a first) have a second for free (they are in a box nearby and they go very fast) 
At one time I would take this same box to our local show and have them priced down. They went very fast then as well Now its the  free box for buying customers at xmas. Its also a xmas good feeeling deal

My way of wording the same info that @Callie Beller Diesel and @Roberta12 have said is:
We have enough problems getting clay and glaze that are sold as compatible, to be compatible, without adding an ingredient that is scientifically not compatible.
You wouldn't dream of putting diesel fuel into a petrol-engined car (or vice-versa), so why would you want to mix clay and glass. 
Petrol and diesel both come from crude oil, but are not compatible.  Glaze and glass may both contain silica, but they are also incompatible.

My way of wording the same info that @Callie Beller Diesel and @Roberta12 have said is:
We have enough problems getting clay and glaze that are sold as compatible, to be compatible, without adding an ingredient that is scientifically not compatible.
You wouldn't dream of putting diesel fuel into a petrol-engined car (or vice-versa), so why would you want to mix clay and glass. 
Petrol and diesel both come from crude oil, but are not compatible.  Glaze and glass may both contain silica, but they are also incompatible.

As previous comments, clay that is born flat wants to return there, as these two pictures show:
 
 
I know that doesn't help for this project, but changing the method of forming the clay in future might help.  I made the next version as a thrown dish, and cut the shape, leaving the up-pointing tail from the thrown rim.  It didn't droop.

As @oldlady rightly says, (the sharp scratchy bits are called boogers in my studio).  I tell students that the finish will never be better after firing, so make sure it's as good and smooth as it needs to be before firing.

Here are plate tongs made from plastic jar covers. (For some reason the image is sideways?) The many holes prevent the lids from holding glaze which then drips onto your still wet plate. The little notches in the lid rim aid in holding the plate edge. Once the glaze is dry I dab a little glaze where the lids held the edge. Generally speaking it enables me to glaze plates without any drip marks. Really appreciated with translucent glazes. 

using a matte glaze can give you very subtle results as these pots show.  i use a beautiful matte glaze on the exteriors of most of my bowls.
if you are interested in making functional ware, please try using a spoon in or on a matte glazed bowl before you invest too heavily in the idea of it being ok as a liner glaze.  you will find that the sound of your spoon scraping the bowl will remind you of chalk screeching on a blackboard.  oh, wait.   there haven't been blackboards in school in ages, you may not be familiar with that sound.
 

This is going to sound terrible.   My best employee quit a few days ago because I thought i had made instructions clear about colors but guess I didn't.   I think I've just reached my limit in being a tactful manager.  Well, she could only glaze for the last year or two anyways.  I'm down to just Me in the studio ... and it's the happiest I've been in years.   I'm going to do all the making and glazing.   I'll have one person 3 days a week out front ... wed, thurs, friday waiting on customers and making jewelry.  That's all I really need out there.      Down 2 full time employees in the back now and I'm actually looking at more profit.   Almost zero production loss and the profit per sale is literally around 75 - 80% vs 20%.     The jewelry and fragrance sales way more than pay for the front studio labor.   I'm looking forward to the next year.   Just ordered 4000 pounds of clay and 8 gallons of glaze.    Plus I just got another 50 pounds of dry glaze.  Going to order my other dry glaze tomorrow.   So there will be no backing down
I'm sticking to this.    The numbers look really good.  Just the thought of only needed to load 2, maybe 3 kilns a week is delightful and I can make a really good profit on that.  Better than before.   I figured out each back studio employee was costing me the profit on at least 1 kiln.   So I was doing 2 .. even  3 kilns just to pay people.     I am so looking forward to this.   I'm actually enjoying the work back there now.   
 

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.