Magnolia Mud Research

Probably need two or three sets of hands to liner glaze that pot!
Maybe
  a) a piece of scrap carpet (or somewhat) on the floor to pad the base
  b) something to lean the pot on so it sits at 45° or so, and can then be tipped up and rolled along the length of, also padded with carpet or somewhat
  c) a long pan to catch the poured-out glaze
Pour glaze in, lean the pot over on the thing, lift the base* to the point where the glaze is about to pour out, roll the pot along the something whilst pouring out!
Voila?
 
*Risk, of course, that the pot breaks on the pivot point, hence, larger and softer could be better.

I don’t have clay formula recommendations, but you might consider making an engobe or slip of some of your remainder to coat exposed/necessary areas of pots made from another clay.
Test First!!

Whiz it in a blender. Either dry then wait for the dust to settle before opening the blender or with some water then pour it out and add a bit more water to get the residuals.

@Pyewackette, +1 for just using a very light spraying of flashing slip.
Also, I would pour the liner glaze not spray it. When you spray inside a form such as a deep bowl or vessel there is a huge amount of glaze blowback coming back out at you. I'ld spritz the inside of the (bisqued) pot with water at the bottom and lower part of the wall where the glaze will be thicker and do a quick pour of glaze of about 1/3 the volume of the pot then very quickly pick it up and swirl and dump the glaze out to coat the inside. If you are using the same glaze for part of the top / outside then quickly dip that area after doing the liner.
I'ld also hold this pot back from your first soda firing. I would suggest taking some less precious pots, take some really good notes on what slips are on each pot, application details, where they are in the kiln etc. Learn as much as you can from the first firing, how other peoples pots turned out and any details you can get about those pots and the firing then apply that knowledge to the slips/glazes/placement/firing of this large one.

I don't throw, but stand when hand-building.  I use the highest table at the community centre, and often work on top of boxes to raise my work, and my benches in my greenhouse (studio) are high too.
I fidget, and am short in the body, and find sitting for most tasks is not comfortable.  I did evening classes working with glass - fusing, copper foiling, leading and was the only one in the class who stood for 3-4 hours.  
The only stuff I do sitting is textile based - spinning, sewing, knitting, although for spindle spinning it's easier to stand, same for weaving.
I think it's important to be at the right height, and comfortablewhatever you're doing.,

I decided to run a couple experiments to test some of the recipes discussed here earlier  Both were fired to cone 7.  I work at a community studio (Duluth Art Institute) and cone 7 was the next firing...I didn't want to wait for the next cone 6. Close enough.
First, I made up a batch of Min's "Base Test." (See Oct 1.) Then I took the same recipe and left out the talc but replaced the amount of silica that the talc would have provided.  In other words, I just took out the MgO.  My test tiles were Continental Clay "B Clay", bisqued to cone 08 and scribbled with a black underglaze pencil.  The difference was not dramatic, but I would say the "Base", with MgO, is more "fluxed" than the modification, without MgO. (Notice how the underglaze ran more when MgO was present.)
Then, I made up a batch of Bill Kielb's "Marcia's Matte", (See Oct 2) and added 10 and 20% of SiO2.  Once again, the results were not dramatic, but the added silica did indeed make the glaze glossier, though still somewhat matte. Notice how the scribbling on the original is a bit less black than the other two.
My conclusion is...yes, MgO can act as a flux in midrange glazes.  
I have to admit that I haven't worked with midrange glazes for quite a while. I've been working at cone 8 to 10 for the past couple of decades.  And I rarely use borates for high fire.  It just seems like cheating!
My original thinking was this: if it's so easy to make a high fire magnesium matte, how can MgO be a flux at a lower temperature?  Maybe borates are the key.  Maybe.  Where to go from here?  Surely there's no simple answer.

since you live in london, home of many excellent museums, i suggest you contact the one whose collection of old ceramics is their main feature.   there may be some easier way that their staff knows about and i am sure someone would be able to help you out.  do not send an email, call first to ask the name of the person who oversees the ceramic and pottery exhibits then contact him/her.  always go to the top first.

I have made a salt kiln out of an electric. The bricks are all rated for 2400 so its a non issue for cone 10 as  Bill points out above

I would ask the person who fires the kilns to put it through the bisque firing as it is (with the glaze on it) then put it through the glaze firing. Saves having to do slow ramps for an entire glaze load.
Welcome to the Forum!

Are you sure it has the correct burner orifices for propane?
With venturi burners you should be able to set the air flaps and leave them, controlling the speed and reduction with just the damper and gas valve.

Definitely a thickness issue. Years ago, I had a student that made animal objects at home with our clay. They brought them in while still wet for me to fire them. looked at the pieces that were about 8 inches in length and 5 in width. Felt way too heavy. So I told them so and we decided on a plan to lighten them up without losing the form. On the legs, she cut long deep grooves and then used a child's spoon to hollow out the inside, then sealing it back up with a slab of clay the same thickness as the leg area, We followed this plan with all the body parts, and found ways to leave hidden air holes to help with drying and firing pressure. Drying was done over a two week period, and firing turned out well along with the final glaze firing. Years ago, but I remember it well . . . . especially the excited smiling face after seeing them being unloaded from the kiln.
 
best,
Pres

It depends on the kind of underglaze you use.  If it is a transparent underglaze then no, it won't work.  The black will show through and kill your red.  If that's the only kind of underglaze red you have, first mask it off and apply an opaque white underglaze where you want the red of the lady bug.  Then apply your red over it and it should really pop and stand out nicely.
Conversely, if you have an opaque red underglaze it might work if you layer it up thickly enough.  It will still probably come out a fairly dark red though.  If you want a bright red against the black, do the same as above by laying down an opaque white base where you want the red to be, then layer the opaque red over it.  This is handy though because you can easily make the white variable.  Thicker where you want the brightest areas to pop, and thinner or absent where you want the red darkest for a more 3d lighting effect.  Like laying in primatura for a chiaroscuro effect in an oil painting.
I hope this helps - have fun with it!  Oh, and then  you can always clear glaze just the lady bug part to a high gloss against the matte black porcelain if you want it to really stand out.  I frequently use different sheens in my glazes on the same piece when I work on my equine sculpture.  Maybe a matte base with a satin horse with high glossed eyes and hooves for example.

When I'm teaching newbies, I always say, nothing thicker than my thumb.  I think thickness and uneveness is causing the cracks during drying.
If you imagine a sphere, say 150mm across, with a thickness of 10mm.  The clay on the outside will shrink and crust over as the moisture evaporates.  The crust makes it harder for the moisture deeper in to evaporate, and as no evaporation, it can't shrink.  So the outside is shrinking and the inside isn't.  Tension pulls something apart and you get cracks.

I guess I'm one of those that unloads the dishwasher from the top down. . . water dripping on other pieces. My answer to this is rounding the inside of the foot ring or cutting grooves in the ring to allow drainage. Just and aside from a practical solution to maybe a non existent problem. .. . Depends on how you look at it.
 
best,
Pres

Bisque makes great slump and hump moulds for handbuilding on, but I think plaster is a better tool for slipcasting. 

All of my teachers in college were pretty focused on form, so we were forever and always looking at feet and rims as important design points in every piece. They’re small details that can either make or break a good pot. The chapters on feet and rims in “A Potter’s Workbook” by Clary Illian are the main reasons I recommend it to anyone starting out making pots. 
These are the 3 profiles I’ve been using the last few years. If a piece is more or less cylindrical, I favour a rolled foot that creates a little shadow underneath the pot, so it doesn’t look like it’s glued to the table. Teacups get a trimmed and altered foot, because they’re supposed to be a little bit fancy. I used to do most of my bowls with the unaltered version of the teacup foot rim, but lately I’ve been playing with creating an upside down S curve on my bowl feet. 
I also will leave at least a little exposed clay on most of my pieces, because I think the clay itself is really lovely, and I like contrasting it with the glaze. Feet tend to be a logical place to do that. 

On plates its got to not chip, same with bowls-no sharp edges. It need to hold the pot off the table and catch glaze if thats in your temp range. It needs to last a lifetime of use without chips.
It take lots of work to get this one figured out and at the same time make it yours.. I always say at least 5 years and thousands of pots. I look back on my earliest work and it substandard foot wise and it was low fire in high school. The glaze did not run but the foot was an after thought not part of the whole work.

Adding one more suggestion, take some of your claybody and make it into a very thick slip.Add a drop or two of Darvan  to about a cup of the slip then brush it through a 60 mesh sieve. Once the pots are leatherhard brush or dip a thin coating of slip over the rims and base/foot. I would do this after  burnishing the grog back into the clay. Since the slip is the same as the body there won’t be glaze fit or colour issues. The grog free slip will give you a much smoother finish.

Good info, thank you for sharing.  Being relatively new to over glazes and having found so very little online to assist me, I've had to dive in and simply use trial and error.  I've been doing under glazing for years but the past three years or so I have added china painting and enamels to my work for color adjustments.  I find I'm using it more and more.  But I have had to wing a lot of it and ask my fellow artists directly for tips who have used over glazes longer than I. 
Speaking recently with the owner of one of the two largest china painting companies in the USA, she sadly informed me that sales are way down and she was gladly giving people like me free samples to try.  She predicted that china painting would be a dead art form in the USA within ten years or less.  Our 'Baby Boomers' are dying off and no one is taking their place to continue with over glazes.  Partly due to the fact that the techniques and information to get started is so closely guarded that new painters have a terrible time getting started.  There are some workshops out there and a few books but as mentioned, they are far and few between and pricy.  
I need to make more blog posts of my own on the techniques and materials I am using.  What little I know is still very useful for others to get started with and from there they can at least do a lot of test tiles and forge their own way if need be - as long as we can still get the materials in the coming years.  How I am using china paints however is far from the more usual traditional methods, I imagine.  Not having any instruction in their use I'm finding my own way and what works for me with horse sculpture rather than dishware.

Use India ink on the larger crackles, and soak them in tea for the secondary pattern. Apply the ink when they’re freshly crazed out of the kiln, and then do the soak a few days later. 

The least intuitive part of collaring is the part where you have to speed up for it to work better. Other than that, some things to keep in mind are to make sure you do a small pull or a pass with a stiff rib in between collaring passes. 

Glaze calc software is for glazes although some functions of it, like oxide amounts can be useful. What is still important is to know the qualities of the materials. Talc for example is good at bringing absorption down if just a few percent are used at midrange but when used at high amounts  it acts as a filler. Also, COE figures are useless as clay is not a glassy matrix.

It depends.....  Some people are heavier appliers than others.  Commercial glaze usually says apply 3 coats.  I have seen people apply three coats that look like my 1 coat, and others whose 3 coats would puddle all over the kiln shelf.
At our community centre, I get newbies to do a "number of coats" test tile, before they do any glazing or underglazing.  A square tile of clay, marked into 4 smaller squares.  Corner 1 gets one coat of underglaze, corner 2 gets 2 coats, corner 3............etc
This gets bisque fired and then they apply 1 coat of clear on half of corner 1, 2 coats on half of corner 2..........
Second fire to earthenware, (we don't do any high-fire) then they (I) can determine if they are a light applier or a heavy applier.  This gives them a guide for future applications.    
We often repeat with a vertical tile with texture.

That glaze does not have any Gerstley Borate. There is no way it would look like that unless you under-fired it. The fact that you can put another glaze on top of it and it is still mostly on the test tile means there is definitely no GB, unless that other glaze on top has absolutely no frit in it?
So if you wanted to replicate that glaze and figure out what it is, I would do this.
1. Since you said you had a few unlabeled bags, I would mix up that exact glaze but missing the 50% GB.
2. Figure out how many cups you want to use, so say its 10 attempts to figure this out. Measure out 100ml of glaze with the mystery recipe but don't include the Gerstley Borate. Instead leave that out. But use the same amount of water you would have used if it was in there.
3. Measure out 10ml with a syringe into each cup, making sure you stir well before each one. I use a milk frother for this...
4. Take your 10 mystery attempt materials that look similar to Gerstley Borate that you might have used instead. Put 50% weight(the missing GB amount) of the mystery chances into each cup. You will have to do the math on this to figure out what this should be, but it isn't difficult at all.
5. Glaze a test tile or a little test dish or whatever you want in the 10 mystery glazes and make sure to record everything.
6. Fire it all on the exact same schedule with the same load mass that you did for the first tile.
7. Pull out the results and keep any of them that look similar to the original tile, go from there.

Just a few drops of dish detergent worked into brush bristles.