Rae Reich

Commercial glaze makers have tuned in to the cone 6 market very well in the past twenty years. You can be assured the claims they make hold true most of the time. I’m impressed to see color charts and samples fired to cone 6 of products originally marketed as low fire. The formulas are proprietary so there’s no way to know if or how they’ve changed them to accommodate the higher temperatures. They “just work.”
Cone 10 is different. In fact, they don’t make many claims about that, other than “it might work.” This kind of firing usually happens in a gas kiln and a reduction atmosphere. Cone 10 firing in an electric kiln is unusual. The atmosphere makes a difference. Finding a different clear glaze may be a worthwhile chore. 
Another avenue to pursue is using slip. The easiest course is to use your clay body and add a significant amount of black Mason stain (6600 is my go to).
Unfortunately, I can’t suggest a percentage because I haven’t personally fired those test tiles yet (Coincidentally, they’re slated for Saturday, 20% 6600, 80% B-mix5. It’s in a soda firing at cone 6, rather different from what you’re experiencing, but I’m still shooting for clean blackness like you. I’ll let you know), fortunately, you shouldn’t have to worry about fit issues because the slip is mostly the clay body, unfortunately, you’ll have to apply it in a wet or leather hard state to be safe, fortunately, you’re doing sgraffito which suggests you’re already doing that, unfortunately, you’d have to mix up the stuff yourself, fortunately if your current clear glaze works with the clay body it should work with a slip made mostly from the clay body, unfortunately, maybe those bubbles were there already and you didn’t see them until they were over a large black surface and it is just the glaze…
Fortunately or unfortunately, this is ceramics. It only gets better. 

True artist. Brava!!! Brava!!!
It’s a special place we have here. Thank you for the saga. Inspiring. 

Don’t fire on drywall it will smolder and burn once calcined. Used to be type X 5/8” - approx. 60 minutes, 120 minutes to sustain a flame. I like the shelf idea and unless once firing, you can grog / silica the shelf after bisque. Many of our large format sculptors would build on a very straight shelf on top of a four wheeled cart.

So beautiful! I really admire the way you approach problem-solving - experience and experiment. It’s wonderful to get new information about the many things clay can do  

To wrap up my little saga on this first ceramic Porthos - he successfully sold at auction yesterday for more than $1600 and I am quite pleased.   Yay!
Alas, I am having issues with his mold so there will be no more of this edition forthcoming until I resolve those issues or make an entirely new set of plaster mold pieces.  I master molded the original plaster mold pieces using silicone but I ended up using a shore hardness 1 step softer (30A instead of 40) and I'm having problems with the new plasters being warped and NOT fitting back together correctly.  Even though the silicone rubber pieces are each in their own mold boxes around the sides (but not the bottoms).  Grr.  I should have known better!  Moral of that story is do NOT be lured by being able to get two gallons of platinum silicone for half the usual price on sale when they are not the correct shore hardness I need.  Close, but no cigar.
Ah well, maybe I can fix the issue by making simple plaster jacket molds for the bottoms of silicone molds so they can't possibly warp from the weight/pressures of the wet plaster filling them.   I think there is a very slight gap there that's sagging so it may end up being an easy fix.  Maybe.   I'll just have to try it today and see.
Meanwhile I have already printed a replacement Porthos of the same size and version to make a new mold from.  I made the first mold set to cast him in pieces for testing my bone china slip I'm making from scratch here in studio but I also want to be able to cast him as close to whole as possible for earthenware production anyway, so back to the drawing board with claying up...  I'm also working on molding him in a larger version so I may well have the big boy casting before I have this smaller version back into production.  One never knows!  Murphy's Law always rules whether I like it or not.

Way cool!! (yep-link works fine) BTW--I tried to find you on Pinterest but no luck--what is the name of your board? I searched for Hyn Patty.

Here is the completed piece all glazed up.  All work has been done in the kiln, completed with satin glaze and minimal china painting.  The only thing on him that isn't ceramic media are his mane flights which are enameled metal, inset with pins into tiny holes along his neck.  Completed to a light dappled grey and with an optional base I have made for him to be affixed to, this piece will be posted to public auction.  He measures 3.75" inches tall and will be a unique color and variety in an edition of not more than probably 20 or so variations.  Once I have finished editing his photos and he has been sold at auction, I'll add a photo to my gallery album with him standing on his base.
Tada!  So now you know how I make my equine fine art sculpture in ceramics.  This one earthenware but I also work in porcelain and fine bone china.
 

I do not have a preferred store purchased overglaze. I spent a whole summer trying to resolve this for our cone 6 glazes so our artists could paint freely with most underglazes. The solution was to modify a clear with a bit more boron to more completely melt. If this is your issue then testing other commercial. over glazes  as well as lighter coats of underglaze  are probably your best bet.
I have never noticed a difference between clays but only anecdotally tested on the variety of studio clays used at the time. Color, thickness applied and even underglaze supplier had the greatest effect - all reasonably resolved with a slightly lower melting temperature of the modified clear. Not all under glazes will go to cone 10 as well with significant color change often being the effect of higher temperature. All our artists, especially sculpture test their underglazed for color. It was not uncommon for a sculpture artist to say, I need to stop at cone 2 - no higher as the perfect color of a full sized human bust that took them a month to make might change from what they wanted. A bit tedious, but an example of all tested in advance.

My hoarding has paid off in spades so far. Out of Kingman feldspar after burning thru 3,000#s in  40 years now I'm using custar instead but have 1,000#s

Custar is back now Dick at least from Laguna locations

If you calcine alumina hydrate to drive off the H2O you will be left with alumina oxide.  (approx 35% LOI from it) Whether or not the mesh size is different I don't know. I found a pdf with mesh size of various aluminas here if you want to compare it to your alumina hydrate. If you don't calcine it there might be issues with both the glaze lay down and/or gassing off (plus the obvious difference of adjusting the recipe to allow for the LOI if you don't calcine it).

The OP shows a location in the UK and price limits in pounds. The standard electric service there is 230V. In the US, standard residential electric service is 120/240V, where the normal household receptacle is 120V and other special purpose receptacles are 240V. Thus, in the US, there are a variety of kilns designed for either 120V or 240V service. I don't think there will be any 120V kilns in the UK, but I could be wrong.

The only thing I am out of is Lithium,  so I don't mix any glaze that contains Lithium.   One of these days they are going to figure out how to make a battery easier and cheaper without using Lithium.     Denice

I have been spending a lot of time and energy replacing my Gerstley Borate glazes. The new ones are coming along, but they just don’t have the same glow and magic. I am trying to push the limits of “how much boron can I get away with?”
The good news is that switching to Fabi Talc in place of Texas Talc has been seamless. 

I've had to reformulate over half of our pottery glazes, both cone 6 and cone 10 to accommodate the end of Gerstley Borate, Custer, and Texas Talc. Using frits for GB, Mahavir Potash Feldspar for Custer, and dolomite/whiting for talc. Of course, all of the recipes needed changes to amounts of silica, kaolin, etc.
The tough ones seem to be the recipes that were "breaking" glazes or used some sort of lithium. Those took some tweaking and testing to get to a similar match to the old glazes.
Our local supplier has switched from Custer to G-200EU in their clay body mixes without any noticeable difference.

The clays I like from "nearby" vendor (about a hundred miles) have been reformulated.
The whitish stoneware seems about the same, the red behaves a bit differently.
My intent was (still is!) to look into alternatives this year; I've enough Gerstley, Custer, and Texas Talc to last a few years, however, years go quickly...

I have a favorite glaze that uses talc, and up until lately have had quite a bit to use in glazes that I bought several years ago. However this last glaze batch found me with none left. Last year I had heard about  a shortage of talc, and believed I would have to reformulate my glaze to work with a different component, maybe dolomite. Imagine my surprise when I found Fabi Talc listed at SC. So I will be ordering some to use in my glaze. However, it makes me wonder if anyone has had to reformulate glazes or clay bodies due to a lack of materials? 
QotW: Have you had to reformulate any clays or glazes due to a shortage of materials?
 
best,
Pres

All the time now. Gerstley is gone and Gillespie is not a perfect match. 3134 requires a complete rewrite of the recipe. Custer is gone from the market, but I still have some. G200EU might be ok, haven’t tried it yet. Still have some old Amtal talc, but some Fabi is in my future. What’s next?

Pictures for sure for ideas. One experience  - Not all clear glazes melt well over underglazes so the underglazed decorated area can be locally refractory. The end result for this defect, bumps, orange peel, glaze that does not cover and melt well to a smooth finish. Often mistaken as over fired and bubbling. Additionally if heavily underglazed, when applying glaze over the top the underglazed area is far less absorbent than naked bisque so it can be hard to apply glaze over the top evenly. Spray applying overglaze and drying is one way to get a pretty even coat over the entire ware.
Pictures for sure here as there can be other reasons for sure. Most underglazes need to be tested with their overglazes in the manner they will be used to be reasonably sure the combination works as intended.

I second looking to glazy.org. Another option is to find a pre mixed powder that suits you. That’s a good middle way. 
If making glaze from scratch, you will have to decide if learning about glaze chemistry is worth more than buying a pint of glaze. Your time and mental real estate are the true costs, raw materials are cheap. And you’ll need a scale. And probably a sieve. A few other things too.

Hard to know the formulas for sure of commercial glazes but there are many sources for glaze recipes. Glazy.org is one web resource. Making glazes for many potters is a thing though, often to save on the high cost of commercial glazes but also often to design glazes that are very durable, to their coloring and finish preferences. You can browse glazy (and others) for something that might be similar.

Laguna now sells a flameware body so you can get a 25# at Laguna/Aftosa location in Fl

Damn @neilestrick!!!
Best low fire work I’ve seen in a long time. So good I could eat it.
To @Biglou13, bisque anything will tolerate that kind of abuse better than a clay close to maturity. Lots of grog, kyanite, or even spodumene (gotten a bit expensive lately) will help. Unglazed flameware sounds better than any other clay body, but I haven’t tried that.
It can be done in the studio, ordinary clay products that handle the extreme temperature swings of cooking, but that’s far from saying it should be. I’ve made several earthenware pots that survive the stovetop, experimenting with clay formulations and bisque temperatures. Though I used them regularly, the maintenance was unforgiving (mold will not be denied if given a habitat to grow), and sooner or later they will fall apart despite all best practices. You didn’t bring this up, but I would never try to sell such a thing. Too complicated. 
Pottery used for cooking is largely disposable, in places it’s actually used. Manufacturers like Le Creuset and Wiliams-Sonoma create ceramic ware that’s functional in a contemporary kitchen, but they are carefully engineered first world conveniences. If people can afford to use metal they do. 
I’ll confess, I’ve never had better beans than those I cooked in clay pots. I don’t want to discourage anybody, I just think they should know what they’re in for. 

After looking at the manual posted in the other thread this controller seems a bit weird. T1 is the time taken to get to the NEXT temperature not the C1 temperature. That seems backwards to any controller I have programmed. 
C1 0020c, T1 0030m
C2 0100c, T2 0030m
C3 0100c, T3 0600m
C4 1140c T4 0100m
C5 1240c T5 0000m
C6 0020c
 
That should go from room temp to 100c in 30min and hold at 100c for 30min. Then it takes 10 hours to get to 1140c (you could speed that part up) and then up to 1240c in 100m (60c/h)

First, why vinegar in the throwing water?
Usually when we see black on the wheel head it is simply a tiny bit of the aluminum abrading from the clay. Could just be the abrasion of the bat against the wheel. Does wiping it down not remove it?