Bill Kielb

Alumina will not develop an affinity to any materials in your clay, your shelf, or your waster slab. It’ll stay a white dry powder.
 

I sprinkle alumina whenever in question. Alumina wax only for lids to keep the alumina in place etc… for me. Yes, anything that sticks can cause issues so common sense try not to let anything stick when in question. Waster slabs can be a nice flat place to fire, maybe flatter than most shelves. There is nothing magical about them when they stick, or your ware sticks to them. In theory they will shrink at the same rate as your ware.
Millions of pieces go through quartz inversion up and down every single day. I mention because it’s really hard to control real cooling rates in an electric kiln and they may be less than you are thinking naturally. With electronic controls, more of a drop and hold function with most kilns, but if you can do it - great! If you can’t, avoiding sudden cooling is a given, don’t open the lid.
I did add a graph of a cooldown rate for a gas kiln below because most folks don’t map it. This is for a very  leaky gas kiln that cools fairly quickly. After 1300 degrees its natural rate is well below 100 degrees per hour ( see the actual column). If you can control cool, have at it, but natural rates may be much lower than you are aware of.
 

When it cracked it stuck which means when it cracked it moved independent of its normal rate. It’s often still much smoother than a shelf so most often fine and the only way but if the ware sticks to the waster and the waster cracks, same issue. A bit more rare, but same result. Alumina both sides works wonders for things that just can’t break. And yes I have seen it happen, especially clock faces.

My direct experience making these - Dragging on shelves — major issue with large discs. It’s not just shrinkage either, everything move a little bit even after fired by the amount of their COE. Something to prevent it, alumina, or silica, not a bed, just enough to smoothly slide on. (Evenly spread Salt shaker if you will). Waster slabs in theory expand and contract same as parent clay, except …… if they stick on the shelf, same issue. Cooling - lots of speculation on this, just never found a lot of validity or physics here. Kilns cool by losses through shell then radiation and conduction. Don’t open the lid, let it cool fully. 
So very flat shelf, something to slide on (alumina, silica), load towards the middle of the kiln if you are worried about cooling too quickly. Flat is less likely to drag or catch than sharp narrow foot. Eased edges - always.
If these discs have a hole in their center such as a clock face, then all bets are off as the drilled hole sets up a very common stress in these pieces and generally requires reinforcement around the hole no matter the material drilled. Google: stress concentrations around holes for a bit ot technical reasoning. 

I sprinkle alumina whenever in question. Alumina wax only for lids to keep the alumina in place etc… for me. Yes, anything that sticks can cause issues so common sense try not to let anything stick when in question. Waster slabs can be a nice flat place to fire, maybe flatter than most shelves. There is nothing magical about them when they stick, or your ware sticks to them. In theory they will shrink at the same rate as your ware.
Millions of pieces go through quartz inversion up and down every single day. I mention because it’s really hard to control real cooling rates in an electric kiln and they may be less than you are thinking naturally. With electronic controls, more of a drop and hold function with most kilns, but if you can do it - great! If you can’t, avoiding sudden cooling is a given, don’t open the lid.
I did add a graph of a cooldown rate for a gas kiln below because most folks don’t map it. This is for a very  leaky gas kiln that cools fairly quickly. After 1300 degrees its natural rate is well below 100 degrees per hour ( see the actual column). If you can control cool, have at it, but natural rates may be much lower than you are aware of.
 

@Bill Kielb Paragon shows the elements as 9.25 ohms each.
@sal.349 If this is true for your elements, you'll need to wire each pair of elements (top pair, bottom pair) in series and wire each pair to a switch at 240 volts, or wire them in parallel at 120 volts. Does the kiln have a 4 prong plug? Measure the element resistance so we can be sure what we're dealing with here.

When it cracked it stuck which means when it cracked it moved independent of its normal rate. It’s often still much smoother than a shelf so most often fine and the only way but if the ware sticks to the waster and the waster cracks, same issue. A bit more rare, but same result. Alumina both sides works wonders for things that just can’t break. And yes I have seen it happen, especially clock faces.

When it cracked it stuck which means when it cracked it moved independent of its normal rate. It’s often still much smoother than a shelf so most often fine and the only way but if the ware sticks to the waster and the waster cracks, same issue. A bit more rare, but same result. Alumina both sides works wonders for things that just can’t break. And yes I have seen it happen, especially clock faces.

My direct experience making these - Dragging on shelves — major issue with large discs. It’s not just shrinkage either, everything move a little bit even after fired by the amount of their COE. Something to prevent it, alumina, or silica, not a bed, just enough to smoothly slide on. (Evenly spread Salt shaker if you will). Waster slabs in theory expand and contract same as parent clay, except …… if they stick on the shelf, same issue. Cooling - lots of speculation on this, just never found a lot of validity or physics here. Kilns cool by losses through shell then radiation and conduction. Don’t open the lid, let it cool fully. 
So very flat shelf, something to slide on (alumina, silica), load towards the middle of the kiln if you are worried about cooling too quickly. Flat is less likely to drag or catch than sharp narrow foot. Eased edges - always.
If these discs have a hole in their center such as a clock face, then all bets are off as the drilled hole sets up a very common stress in these pieces and generally requires reinforcement around the hole no matter the material drilled. Google: stress concentrations around holes for a bit ot technical reasoning. 

A memorable quote:
 
 

Wow. 20” diameter and 3/8” thick, that’s no mean feat, in my book anyway.
Of the things you listed, two: sand on the shelves (a dusting, not a bed) and posts around it are things I’ve heard before. I agree that a “cookie” or waster slab presents some new variables and, I believe, unnecessary complications. You mentioned alumina and wax, and that reminded me of a workshop I attended with Tom Coleman and Paul Geil. They had a wonderful presentation, Tom throwing beautiful porcelain pieces and Paul explaining how to fire the kiln. Anyway, the porcelain Coleman used would fuse to the kiln shelves and pluck so the thing they did was to dust the shelves with straight alumina. If my memory is correct it was put in a  little sieve, like a sifter, and Tom just tapped it until the shelf was dusted. It’s very much like the sand idea, but alumina is more refractory and guaranteed not to stick. I’m not a fan of making extra dust anywhere, but this seems like an exceptional situation and not something you’d do day in and day out so I don’t consider it risky from a health standpoint. 

Do you have two of those switches or just one, and can you confirm the wiring diagram I posted matches the wiring in your kiln?

This may help, an article on the forum from 2020 and a schematic.
 

It’s really no different than adding insulation to your house. Adding double reduces losses to ~ half. R value of materials are basically additive for homes, kilns are similar. How it’s done is key so if just building a kiln no worries. If retrofitting then removing the steel shell easiest way. The lid is also an issue as over heating the already banded bricks can cause larger deflections and lid failure. Easiest improvement for the effort I have seen is Add 2” rigid to the interior of top and bottom where maximum heat loss exists anyway. But to answer your question, more insulation, less power to make cone, but also slower to cool down.

Since this does not appear to have a sitter or shutoff device it’s quite old and likely needs to be fully supervised while firing. I would not operate this until it’s been inspected and confirmed safe, confirmed it will meet your needs for the cone you fire to, and the cost to improve it makes sense for those needs.

Since this does not appear to have a sitter or shutoff device it’s quite old and likely needs to be fully supervised while firing. I would not operate this until it’s been inspected and confirmed safe, confirmed it will meet your needs for the cone you fire to, and the cost to improve it makes sense for those needs.

I have tried several, some with an air brush. Kinda ok, not sharp like I wanted, really hard to keep it from creeping under the  surface for me. Sticky, stretchy from Crickut machine seemed best. 
Best stencil idea not entirely related was real pin stripe tape, exacto knife, make your own stencil, then cover adjacent and chemically etch the existing clear glaze for custom personalized mugs including personalized painted scenes. Those were pretty pricey or very custom appreciation gifts that needed to leave with zero flaws. Could not find any stencil combination that did not have too many minor flaws. They were meant to be premium quality, porcelain, sign artwork on mug ……… never want to make those ever again!
The automotive pin stripe tape held very sharp lines throughout the entire process though, best I have tried, but lots of work put into those mugs.

I have had my best luck with stamp pads and a well cut stamp. If it can be symbolic such as three dots …. Etc it’s far easier than a scripted name. Picture below is one of our test bowls, stamp is a bit inconsistent. It takes significant effort to get it perfect and has to be rolled evenly to match the shape when stamping. So smaller and maybe symbolic is far easier.

Seems like even more trouble - never tried decals

I have tried several, some with an air brush. Kinda ok, not sharp like I wanted, really hard to keep it from creeping under the  surface for me. Sticky, stretchy from Crickut machine seemed best. 
Best stencil idea not entirely related was real pin stripe tape, exacto knife, make your own stencil, then cover adjacent and chemically etch the existing clear glaze for custom personalized mugs including personalized painted scenes. Those were pretty pricey or very custom appreciation gifts that needed to leave with zero flaws. Could not find any stencil combination that did not have too many minor flaws. They were meant to be premium quality, porcelain, sign artwork on mug ……… never want to make those ever again!
The automotive pin stripe tape held very sharp lines throughout the entire process though, best I have tried, but lots of work put into those mugs.

I am thinking the confusion arises from the lack of explanation of cones. While their chemistry is established they indicate by bending so not really fully melted. A cone 4 cone ground up and fired to cone 8-10 turns out to be a fully melted glaze. At present there is not a straightforward exact predictor of when a composition will fully melt. Stull did his work at cone 10 I believe (likely closer to cone 11 today). His map was for a very specific flux ratio and his glazes. Katz showed a reasonable correlation at cone 6 and lower further incorporated boron as a means to lowering the melting temperature. Sue McCleod presented a cone six version of this at a2018 NCECA (Google Sue McCleod and Stull. Stulls map does not predict the temperature something melts at. Higher melting temperatures would progress from left to right and upward in relative ranges. Limits are these tested ranges, but are approximate guidance Testing is generally the only way to know with reasonable certainty when something will fully melt.
Advanced knowledge of durability is a Katz thing where certain flux ratios may be an indication of durability.
I think the simple answer is cones fully melt 4-5 cones higher than their rating. I don’t think the author clearly represented this in the cited link.
I would also suggest that much of pottery is understanding the variability and by experience important trends. It is somewhat very exact and scientific but verified through testing.

Looks like a small well powered kiln. Depending on reduction schedule and firing speed restrictions of the wares you will fire likely 8 - 10 hours. If your wares are not particularly thick and you can go 400 - 600 degrees per hour with let’s say a 45 minute early reduction maybe gets you a bit faster down to 6 hours. This kiln looks like it can go to cone 10. One thing about small updraft kilns folks are prone to stalling them in very heavy reduction. The supervision of damper and gas is often something to get used to and too heavy a reduction will drop your speed a bunch. In my experience most folks tend to stall these kilns on their way to cone 10 so cone 6 reduction should be easier to hit.

Hi AndrewB, Welcome to the Forum!
Is that a "flange mount" bearing assembly?
If so, is there a cartridge bearing that is replaceable, or must the entire assembly be replaced?
Bearing 6204 is familiar to me; it's common for bicycle hubs. I've used the double seal (designated by "2RS") type.
ABEC 5 seems a reasonable tradeoff between cost and durability, a worthwhile step up from ABEC 3, imo; I'd stay away from the "ceramic" type - high cost for a bit less friction and not necessarily more durable...
The Enduro brand is widely available and lasts better than other brands I've tried. Vendor WheelsMfg.com has been great ...they used to offer cartridge bearings via https://bicyclepartsdirect.com/ - however, I'm not seeing 6204 (or any other wheel/hub) bearings there now...
 
The 6204, however, is 20mm ID x 47mm OD x 14mm wide.
I'm not finding a spec of BPF6X30MM
Added: if there is a cartridge bearing to press in/out, a bearing press can be very handy, for
  a) it is important to apply force only to the outer bearing race edge (of the new bearing)
  b) it is important to maintain alignment throughout the pressing (in and out)
Once the bearing gets a bit tilted, it can be difficult to straighten out without damaging the bearing and/or the hub.
 
Added II: does it look somewhat like this?

I wanted Glazy’s target and solve for something, so I sprang for the membership for a year. The paid version lets you overlay the common limit formulas over the Stull chart. It also lets you overlay the Montmollin fuse charts, which is less helpful, but I digress. 

The limit formula overlays are a nice visual. If you’re not at the place where you want to pay a bunch of money for glaze software, I’m happy to screenshot here for educational purposes. 

Congrats!