Magnolia Mud Research

There had been discrepancies in Orton's online materials regarding small cone deformation temperatures*.
I used small cones for the kiln sitter (prior kiln), and I like them. They are a tiny bit cheaper.
Here's an excerpt from the message I received from Orton Engineer (circa March 2020):
Dear Sir,   Thank you for contacting Orton to draw our attention to some incorrect information on our website.  The small cones do in fact deform after the large cones.  We have updated the information on the website.    See also this thread (start at the bottom/end) Kiln Sitter cone for witness cone - Studio Operations and Making Work - Ceramic Arts Daily Community   *There still are! The 2016 cone charts are all "backward"   See the small cone page, SMALL CONES | Orton Ceramic
"Small Cones used on the kiln shelf deform at about 9°F after Large or Self-Supporting Cones of the same number." It's just because the small ones are smaller - the cones are same material; same reason a large candle will slump in hot weather before a small one.
 

Further to the technical explanation in the link, I put it like this to my group.
Underglazes (usually) don't move or mix during firing.  They sit/stand still.  When clear overglaze is applied to fired underglaze it (usually) doesn't cause the underglazes to move or mix.  And (usually) the colour is WYSIWYG.   So, underglazes are good for painterly pictures, drawing, writing.
Glazes can/will move or mix during firing, particularly on vertical surfaces. And (usually) they are not WYSIWYG.
So, if you want defined colour areas, use underglaze, if you don't mind if they mingle use glazes.  Apart from all the other options, like matt, textured, crystals.........

Some cracks never heal and some get bigger. Learning which are fixable takes experience . Yours is only going to get worse and if it where mine I would toss it and make a new one.I have used bisque fix for many successful repairs but knowing its limits will take time for anyone to learn

Could be the glaze layer at the rim is thinner.
Could be the rims were worked more at finishing, hence sand and/or grog are more exposed.
Could be a combination of both?
Reglazing and refiring, hard to say; some have luck with that, some don't.
Any road, building up a slightly thicker layer of glaze at the rim might help.
Using glaze that doesn't move/sag much might also help.
I'm curious if the problem mugs are together in the kiln, perhaps a hotter area?
I've used a buff stoneware that weeps tiny droplets when fired hot. The droplets stick but can be scraped off bare clay (e.g. the foot ring). The droplets will be hidden in the glaze layer, where the glaze is thick enough, else there's a gritty feel.
It's annoying! ...haven't seen it in any other clay, just the one buff stoneware.

i have given this recipe to members of this forum.  there is a very easy to use red clay made of only 2 ingredients so everyone was  positive it was not good.  someone did the chemistry and discovered it has all it needs.     it is really very plastic, no grog, strong stoneware, pretty red.
try it  50 pounds redart, 50 pounds XX saggar.   water.   makes a lot of great clay.   i no longer use colored clay,  just white.

could it be impending bankruptcy?

That is the exactly what lab testing does. If you follow the link I posted above for BSC you can see how they request a glaze fired sample pot, this is what they test, the fired glaze. They test for whichever oxides you pay for. Example would be if you add some stain that contains vanadium you request they test the leachate for vanadium release. It is the vanadium oxide they give you results for. Since in North America only lead and cadmium need to be below certain government approved levels on dish ware the other oxides are usually compared to drinking water levels. 

Many potters, most likely the overwhelming majority of us, either use a liner without any colouring oxides to be of concern or use one with very low levels of a colouring oxide. I have had a few glazes lab tested at BSC. 
 

Just a country boy here…
What do colorants do in a good hard (stable) glass? Stable meaning it is durable, resistant to both acid and alkali, and smooth enough microscopically to not grind away forks and spoons (cutlery markings).
“Welcome to ceramics… again.” A phrase I repeat to myself as I learn new things I thought I knew well.
If you have a strong well functioning clear liner glaze that is durable through acid (lemon juice/vinegar) and alkali (many dishwasher cycles), stays shiny, and fits your clay, then you've arrived. Zircopax will make the glaze tougher, not unlike alumina, and whiter/more opaque. No additional toxicity. Iron will alter the color and, depending on the %,  temperature, and atmosphere will, produce a great variety of effects. No additional toxicity there either. Chrome, copper, cobalt, manganese, and others rely on the hard durable glass to keep them locked in the matrix. Personally I keep those below 2%. Your worries about Mason stains are in the same vein, except encapsulated Mason stains are designed to be non leaching  in nearly all circumstances. 
If you aren’t confident in how good, hard, or stable your base glaze is, no rules of thumb will help. It is not beyond your ability to find or compose a stable glaze and that should top your to-do list.
Another line of thinking is to put all color underneath a stable glass, as in slips or underglazes. The foolproof option.
No matter what, none of the materials we ordinarily play with holds a candle to lead in terms of toxicity.
Our biggest toxic risk is mixing the glazes carelessly. 

Drifting off topic ...
I'm starting to get confused, because there are two ways of measuring the density of powders.
Bulk density is the weight of the powder divided by the volume it occupies (that's both the volume of the particles and any entrapped air).
Obviously this can be influenced how the powder is handled before the measurement.


Particle density is the weight of the powder divided by the volume of the particles. This volume can be measured by adding the powder to a liquid and observing change of volume. [The liquid must obviously wet the powder well.]
I'd always sort of assumed that heavy & light ZnO differed in bulk density rather than particle density.
Can anyone clarify things?
BTW references to light and heavy ZnO seem thin on the ground. But this supplier seems to be offering 98% pure heavy ZnO. Which suggests that "heavy" doesn't relate to density variations due to contamination with other elements (but may relate to the production process used.)  The listed "density" for this "heavy" ZnO is 5.606, wiki gives 5.6 for ZnO.
https://ie.vwr.com/store/product/787678/zinc-oxide-heavy-98
PS An earlier glazenerd posting describing the two processes in more detail.
 

I'll post this in the pinned Custer thread also.

 

Hi and welcome to the Forum!
If you don't like the results you have so far then there isn't really anything to loose by trying the wash. As to whether it covers the glaze, probably not but something might happen, what happens is going to be trial and error. Washes can be applied then wiped or sponged off green or bisque or they can be brushed on either on top or under a glaze. I would suggest warming the piece(s) up before trying to apply the wash. If you use a copper oxide wash overtop of a glaze it would be best not to do this on a surface that could come in contact with food.
Re your muddy underglaze colour with clear glaze, this can happen depending on the chemical composition of both the underglaze and the glaze insofar as the reactions between ingredients. For example green underglazes that use chrome (in the stains within the underglaze) will turn the green to a not so attractive brownish colour if the glaze contains zinc, chrome tin stains (for some pinks, reds and purples) will get bleached out if the glaze doesn't have really high calcium levels causing grayish tones to the underglaze. Colours can also be affected if the glaze is applied too thickly, underglaze can look milky and clouded.

As a gas potter for 50 years I see the burner is pushed into the kiln  to far-needs to be away from kiln a bit and the air flaps are closed down to much (open them up)
these are the most obvious ,but kiln design and orifice size are also concerns -I can see the 1st two issues in your photos for sure so they are easy to fix.

Thanks for answering a couple points, makes it easier to give suggestions when we have more info. 
I would glaze fire the tiles on waster strips of clay. These don't need to be bisque fired if they are bone dry. For waster strips I would roll out a slab of clay (same clay as the body) to about 1/8" thick and about 19" X 16" then cut it into strips about 2" wide. There can be gaps between strips but have the edge ones fairly close the edge of the tiles. Make the strips a bit thicker if you plan on glazing the edges of the tiles right the way down with no wax line. Extruded coils would work too. Reason for the waster strips (or coils) is twofold, one to allow the tiles to shrink / move freely without getting hung up on the shelves and secondly to allow airflow under the tiles. During cooling the shelf is going to retain heat much more so than the air will, this means the bottom of the large tile can be much hotter than the top surface which can lead to cracking.
I would place the tiles in the middle section of the kiln. If you use half shelves make sure they are lined up perfectly. I would also suggest ringing the tiles with either other pots or square or triangular kiln posts laying on the side, this also helps even out the heating and cooling.
I know you said the tiles are time sensitive so you might not have time to dry waster strips / coils, if so some people have success with using silica sand under large slabs, sculptures also.
To be super safe you could also add a slow cool through the quartz inversion zone. (slow from 1100F - 1000F, tc's measure air temp not shelf temp so the 100 degree range is pretty safe) Don't open the kiln until it has cooled down well below 400F. I would also glaze fire them slowly, not a fast firing.

Good advice so far...I'll just weigh in with this: one big difference between a soft bisque firing and a normal one is that the pots are more fragile. If you use glaze tongs you need to be especially careful not to grip the pot too hard. Super easy to go thru the clay wall or shatter the piece entirely.
Tongue method - in a pinch it works however I prefer to lick my finger then touch the pot. Bisque pots always seem to have a slight dusty/salty surface that seems a little unpleasant.
As mentioned, an underfired pot will absorb more glaze than a properly fired one. Sometimes wetting the pot with water, a gentle sponging, will counter the very dry surface.

Clay is made from materials mined in the earth-they vary ,stuff is in it ,things happen. A few rocks or wood or whaterver is all part of it.

If the colour is the same in both (I'm guessing it is if you haven't noticed a difference) then put a drop of water on a piece that was for sure bisqued to 1860F and time how long it takes to soak into the clay. Repeat with the clay that was possibly only bisqued to 1200F and see if takes longer. You can also stick your tongue on both pieces, the lower fired one will stick more. (maybe don't do this in public)

I put the clay pad on a bat* when using unholy bats*!
*likely the practice is very frown worthy...
 
*plaster and oversized

This thread is bounced off this one that was discussing extruders then went off on a bit of a tangent. I brought up the subject of thrown handles as another alternative for @Pyewackette . (other options such as cast, press molded etc in the linked thread also)
Super quick demo photos below, hope this helps Pye.
Throw a donut on a batt, for mug handles that need to be straight use as big a batt as you have or throw on the wheelhead if it's wider than your batts. Center the clay then open it up right down to the batt.  While throwing the donut press down firmly on the clay as you are pulling it outwards from center. 
Clean up the edges of the donut and shape into a dome top, flat, ridged, whatever profile you need. Can use  rib made from an old credit/gift card to make identical profiles for the donuts if making a lot of one style that need to be the same.
Cut into the donut with a pintool then slowly rotate the wheel to cut the donut off.
At this point you can either let the donut set up a bit or work it now. Cut off a section and either hold it up and smooth the  cut edge or let the sections dry a little then lay them facedown and smooth the cut edge. 
For mug handles pull them straight and let them hang until they are dry enough to attach. 
   
   

Calligraphy style brush. And practice. 

my assumption is that you are looking for the amount of sodium in the slip.  the amount of sodium in:  sodium bicarbonate is Na-H-CO3  (molar mass = 84).  sodium carbonate is Na-Na-CO3 (molar mass =106) therefore:  
to get the same amount of sodium you need to 168 grams of baking soda to get the same amount of sodium if you used 106 grams of sodium carbonate. 
In other words you need 1.58 times of sodium bicarb instead of than the amount of sodium carbonate in your slurry recipe. 
All of this is based on the assumption that both the sodium carbonate and the sodium bicarbonate are "dry" solids; that is they have no water attached to the molecule.  Sodium carbonate is more likely to absorb water than baking soda but both can /and will/ pickup water and still be a "dry" solid.  Both are white powders that will absorb water vapor from the air if not stored in a sealed container.   
I have no useful insight if the reason for using sodium carbonate is adjust the pH of a slurry to control suspension of the solids.  I seldom care about suspension as I apply "paste" instead of "slip" on surfaces.  I add the sodium to get the effect of the sodium in the fired surface.  
Regarding @PeterH question: yes. long ago I began using a water spray of sodium carbonate as a means of controlling the fired effect of surfaces.   
I ran out of sodium carbonate but had baking soda in the cupboard and  used it instead; saved a trip to the store and the results were equivalent.  I have been using baking soda more often as it is less damaging to my skin and is available at the grocery store near home.  
A footnote: once the sodium carbonates are dissolved in water they are no longer "sodium carbonates" but are sodium ions and carbonate ions.  the pH is determined by the total collections of ions in the liquid phase.  Soluble ingredients other than the sodium carbonates also must be considered.  
LT

This!   I noticed many people at the new studio I am at, many are sanding pieces with expensive diamond pads as a matter of routine.  i was taught to fix roughness early on,     sanding  finished pieces is a last effort 

99% Dipping, with some squirt bottle use on pots of a certain size. I don’t have the facilities or equipment to spray, and dipping gets a quick, easy application. I make enough stuff that brushing would be very impractical.
 

You should not be paranoid. You’re likely to have more exposure to free silica taking a walk on a dry breezy day than sanding greenware for five minutes over the sink. No harm in being cautious, but you have nothing to worry about.
A thought for future reference, I like to dampen things with a sponge then scrape using a steel rib if I need to remove material  
 

And don't sand over the sink!  That will plug up your drains in no time.  Sand outside if possible.  wear a mask and just be aware of the negatives of dust.  Lin