dhPotter

A 15-20 minute hold equals 1 cone hotter, so yet, the hold will increase heat work on the rest of the kiln, too. I would first try loading the middle of the kiln tighter and loading the top looser. Put low mass pieces up top, like wide bowls.  If that doesn't work, I would try firing a cone lower and holding for 15-20 minutes to achieve the next cone.

That is how I do it. I demonstrate then let them drive the wheel. I sit next to them coaching for the first 2-3 times they do it. If they need more demonstrating then we switch seats. Then its back to coaching them from the sidelines. Works out well so far. Been doing it this way for 3 years.
 

How do you flip the slabs while they are drying on the drywall? Do you pick them up with your hands, or use another board of drywall on top to flip them - like a sandwich of drywall with slab in the middle?

On the topic of your body being able to do the work: There's a very real possibility that one's business model will have to change (or even stop) well before you reach retirement age. I am at a point in my career where that is happening to me. I'm 51 years old, and after 30 years of making pots my hands hurt way too much to continue wheel throwing. It's nothing that surgery can fix for the most part, and I want to be able to use my hands when I retire some day, so I am now a hand builder! If I was making my living 100% from making pots, that would be a severe change in my production output and my business may or may not be able to survive that kind of change. Luckily for me, selling pots is only one of 4 income streams I have. That said, kiln repair work is also really hard on my body and I'm starting to see signs that I will probably have to stop doing it well before retirement age. None of this was an issue when I was 40, I'm just worn out. Once it starts happening it seems to build quickly, so you have to plan ahead for those changes. There's a reason you don't see many 64 year old guys framing houses or installing roof shingles.

+1 for this. COE values are extremely helpful when adjusting a glaze by increasing or decreasing one or more ingredients that already exist in the glaze. Throw in another material or look at a different glaze recipe and you now are working with an entirely different glaze with its own dynamics. 
Can't just look at one number or range and expect all glaze formulas within that range to fit. In addition to the different oxides contributing different properties and melts you also have to look at the type of glaze it is in regards to both opacifiers, colourants and cooling.
If the Spearmint or any of the other glazes that are matte or semi-matte  then COE numbers are not valid at all as some of the matting agents (like the calcium in Spearmint) will precipitate out the glassy matrix onto the surface of the glaze causing the semi / matte surface. By the calcium doing so you have created a microcrystalline glaze, COE calculation figures only work with a fully melted gloss glaze. Same reason why looking at a clay body recipe in glaze calc the COE figures means nothing given that clay is a crystalline material and not a fully melted gloss glaze.
Re colouring oxides and opacifiers. Some glaze materials, like zirconium, will give you a higher calculated COE figure but this is only looking at theory and not practice. In reality zirconium does not enter the melt therefore the change it imparts in the COE figure is skewed. Zirconium particles act like miniature boulders in the glaze and if a craze line starts the zirconium boulder stops the craze from propagating.   Many colouring oxides and titanium / rutile will also decrease crazing. Boron is another good example of how glaze calc doesn't show the entire picture, in theory you could increase the boron to really lower a COE but in practice if you go beyond a certain point crazing will increase. I would suggest looking at the properties of each glaze material and what the contribute to the glaze, look at COE figures for each of the materials. If you see a formula with high amounts of sodium and or potassium chances are it's a high expansion glaze that will probably craze on a low expansion clay. Look for recipes with low expansion fluxes like magnesium, lithium, moderate amounts of calcium plus good levels of silica and alumina versus ones with higher expansion fluxes and low levels of silica and or alumina and for cone 6 boron around 0.15 
 
 
 
 
 
 

@dhPotter, newer analysis of Custer has fluxes a fair bit different, might be an idea to update your materials data field if you get a new supply of it. Link below to a thread with updated new analysis. Could save some shelf grinding with runnier glazes. 
 

Did you sieve the glaze when you made it up 3 times or just before you used it? Glazes containing soluble materials (including boron frits) can form bits (scale) on the sides of the buckets and in the slurry over time. Other materials, like wollastonite can agglomerate causing "bits" in the glaze slurry. If you use hard water this will exacerbate the solubility problem. Image below from digitalfire that looks an awful lot like your problem, cause was soluble materials causing scale bits. #1 for using an 80 mesh screen.

I would take a Dremel tool and grind them out then apply a small amount of (screened) glaze to those bare areas and refire to a slightly cooler cone. If you went to ^6 before try just over ^5.

+1 with Bill.
Prepare 12 small balls of clay.
Centre without coning. Check your key points.
Seat close to and level with wheelhead.
Elbows anchored, hands working together at all times.
Your hands need to be working  about position "5 on the wheel clock"
Be aware how your hands leave the pot after each pull.
Clean the area where clay meets the wheelhead anytime it gets any gunk collecting.
Throw  12 cylinders.
Cut them through cylinders to check.
 
https://youtu.be/7lllCkLbJFE
Beware of the buttress one potter said... in these forums. It id a thick area which can cause probs..it occurs if clay is not picked up right where the clay meets the wheel head.
Maybe pics of your offcentre pots, or watch a video of yourself working.
Keep at it, observing the key points.
This frustration will pass..pottery life is full  of them, join the mob!

But they should!

Special or custom orders tend to come back and bite you. 
Very much agree with this statement. I now understand why y'all have said over and over to never take custom orders. It is a never-ending saga.

Special or custom orders tend to come back and bite you. 
Very much agree with this statement. I now understand why y'all have said over and over to never take custom orders. It is a never-ending saga.

I used to tell people I would need their first born as a deposit. Now I just say no, I have enough kids. 

Special or custom orders tend to come back and bite you. 
Very much agree with this statement. I now understand why y'all have said over and over to never take custom orders. It is a never-ending saga.

Use your cone 6 clay instead of the lowfire and make a coordinating glaze that compliments rather than exactly matches the lowfire one. A vitrified saucer for a lowfire pot.
Special or custom orders tend to come back and bite you. 

I hate them. My wife and I have a running joke that I will divorce her if she takes another custom order.

How hot did the first firing get? Chances are you shut it down during the period where the glazes are starting to melt and bubble, which can cause crawling. If this happens in the future, stop the kiln, change the program, and start it immediately without letting it cool down.

On my small container shelf  I found Nickle oxide,  Iron Chromate and  White Lead.   I will put them in the cabinet until our next trip to hazard waste.    When I am glazing I wash my hands and equipment in a separate dish pan.   I let it dry up and put it in a container,  when it is full I take it to hazardous waste facility.   My husband and I drove through a damaged area close by yesterday.   Some houses were completely gone, others had the kitchens left standing,  if garage doors were still there the fire department painted the number people who lived there,  if they got out and were OK.   When you live in Kansas you have tornado season every spring and hide  in the basement.   You never get use to the sheer force and damage of a tornado.    Denice

Jumping back in on this latest wrinkle in the question... As dh notes above, the technically proper representation is 1.46, as the technical definition of S.G. is the ratio of the weight of the slurry to the weight of an equal volume of plain water. 100ml of water weighs 100g and (in this example) 100ml of glaze weighs 146g. So, 146÷100 = 1.46. If you are using a 100ml syringe to weigh your glaze on a digital scale, the number on the screen is the S.G. after mentally shifting the decimal 2 places to the left. I would guess that some might just say ".46" because the 1 is implicit for glaze slurries that are heavier than water, or maybe call it 146 because the location of the decimal isn't that important. 

But moving on to the 2-digit number "46", you may find yourself in deep doodoo. In the mid 1760s, a French pharmacist named Antoine Baume "invented" a hydrometer scale for measuring the density of his liquid potions. That scale was based on the density of salt water, and is generally expressed as "degrees Baume". By whatever coincidence, 45 degrees Baume is 1.45 specific gravity. However, the rate at which the Baume scale increases or decreases is different than the rate at which technically proper S.G. changes. Consequently, terra sigillata at 1.20 specific gravity is about 24 degrees Baume, and casting slip at 1.80 specific gravity is about 64 degrees Baume. If you are using a commercial hydrometer, it may or may not have both scales marked in it. I have several hydrometers that I never use (because 1. they are notoriously inaccurate for mid-fire glazes, and 2. the 100ml syringe is so much easier) that has both scales marked on the insert. But one of the large community studios that I teach at has 2 hydrometers sold by Laguna Clay that are scaled in degrees Baume only. The glaze recipe book there specifies a 2-digit number as the target "specific gravity" for each glaze. As long as nobody enlightens them about the technical truth behind that number AND they continue to use the same hydrometers to measure their glazes, they will be okay. But heaven help anybody who takes that recipe and supposed specific gravity somewhere else.

Just to add, it’s a way to achieve more repeatability / dependable applications for each particular glaze.

Glazes can deflocculate over time, sounds like this might be what happened. Soluble materials, such as sodium, can in time leach out of nepheline syenite, spars, frits  etc and cause hardpanning.  Mixing it up more frequently wouldn’t have prevented this. 

I’ld pour the liquid off the top of the glaze, don’t throw it out, and cut out the hardpanned glaze out of the bucket with a loop tool. Remix it  with the liquid you poured off plus some water first and a small amount of epsom salts solution and see if that fixes it. Add enough water to get it back to the specific gravity glaze manufacturer recommends. Might need to add bentonite slurry if Epsom salts solution alone isn’t doing the trick. Given that we don’t know how much clay and/or bentonite is in the recipe it might not need it. 
If you do use bentonite you need to slake it in water until it absorbs the water, it will take a few hours. 
John Britt video of the process here if it helps,
https://youtu.be/0S_gbVkq378

And here are the final photos of the mini 'Hadrian' custom glazed equine ceramic to a rich carmel dark palomino with satin glaze.  I think his owner will be very pleased with how well this commission came out.  She picks him up this weekend!  Thanks for following along and I hope it wasn't too boring.  I'll probably play with these final photos a bit more to brighten them up since they are a touch dark.
 

When those bubbles wear thru that means glass (glaze) came out into fluids most likely witha spoon stirring for example.Not a good situtaion for anyone. If the glaze is to thick toss it as its unsafe for use.  In ceramics we all make mistakes and try to learn from them.It takes a whole lifetime of ceramic mistakes and then in the next life you get to make more-its the learning from them that makes us better potters

Different methods of making handles for mugs https://tinyurl.com/yckrtf6r
Video: https://tinyurl.com/mrnwfa5b

Show up. Especially when you’re scared, or vulnerable, or avoiding it.

If you don't know how refiring is going to effect the glazes it's a good idea to fire them on a thin waster slab of clay to catch any possible glaze runs. First glaze has already melted so refiring it can make it more fluid. Waster slab doesn't have to be bisque fired first, just make sure it's really dry. Another thing you might want to consider is placing the reglazed pieces in a cooler spot in your kiln. Some claybodies (especially porcelains and smooth stoneware) can bloat when fired a second time due to the body being fired with the extra heatwork. Refires are always a crapshoot but if there is nothing to loose worth a go. 
Welcome to the forum!