Callie Beller Diesel

No need for that keep asking questions and doing pottery as you will learn more and we can help

Don’t get too hung up on specific SG numbers. The number that works is the one that gets the right application of glaze YOU want on your pot, and that might be different than what someone else recommends. The 1.4 mark is a rough starting point, kind of like seasonings in a recipe. Some people will want more, some people will want less. 
First, water is the easiest thing to adjust in a glaze, and adding things like epsom or darvan to adjust how a glaze flows should be done only after you’ve determined it’s really necessary for getting the right glaze application. If the bucket has a layer of glaze materials in the bottom within 5 minutes of mixing, pulling out the epsom salt solution is the right thing to do. 
Adding epsom salts will flocculate a glaze, or make it gel slightly. This is why the glaze now seems thicker than it did, and it may leave a more generous layer on the pot than the same 3 second dip would without the flocculation. You might notice on some glazes it will start to crack while drying, potentially causing crawling in the kiln. And yes, it will also slow your drying time. That slowed drying can help smooth out application based drip marks. If your glaze is one that might be subject to crawling due to application, adding water will help resolve that. But know that adding water to a glaze that’s already flocculated will further delay drying. It may mean you have to adjust your workflow to allow for that, but it’s worth it if it means the glaze goes on the pot nicely. 
Another question worth asking is whether or not any of your ingredients are soluble in the bucket over time. It’s usually more of a problem with boron containing materials, but some feldspars can be subject to this as well. Solubles can affect glaze application if they cause additional gelling. 

Don’t get too hung up on specific SG numbers. The number that works is the one that gets the right application of glaze YOU want on your pot, and that might be different than what someone else recommends. The 1.4 mark is a rough starting point, kind of like seasonings in a recipe. Some people will want more, some people will want less. 
First, water is the easiest thing to adjust in a glaze, and adding things like epsom or darvan to adjust how a glaze flows should be done only after you’ve determined it’s really necessary for getting the right glaze application. If the bucket has a layer of glaze materials in the bottom within 5 minutes of mixing, pulling out the epsom salt solution is the right thing to do. 
Adding epsom salts will flocculate a glaze, or make it gel slightly. This is why the glaze now seems thicker than it did, and it may leave a more generous layer on the pot than the same 3 second dip would without the flocculation. You might notice on some glazes it will start to crack while drying, potentially causing crawling in the kiln. And yes, it will also slow your drying time. That slowed drying can help smooth out application based drip marks. If your glaze is one that might be subject to crawling due to application, adding water will help resolve that. But know that adding water to a glaze that’s already flocculated will further delay drying. It may mean you have to adjust your workflow to allow for that, but it’s worth it if it means the glaze goes on the pot nicely. 
Another question worth asking is whether or not any of your ingredients are soluble in the bucket over time. It’s usually more of a problem with boron containing materials, but some feldspars can be subject to this as well. Solubles can affect glaze application if they cause additional gelling. 

I was doing a bit of reading and stumbled across a new Stull chart. He seems to be doing the same experiment but at cone9 instead of cone11. It is interesting that the OG Stull chart is cone11 but everybody uses it at any cone.
This time he is glazing tiles and making the glaze into cones to see the deformation temperature. The solid line AB is his same “best gloss line”. The dashed line CD passes through the lowest deformation temperature in relation to Silica and the dotted line EF the same for Alumina.
 

 
He then goes on to plot it on top of the original Stull chart but misses out his new gloss line so I have done my best to add it back in blue.
 

 
It’s interesting how different the best gloss lines are. I tried adding on Silica:Alumina ratios but I haven’t had any good ideas yet. The wiggly silica line and shape of the deformation eutectic look like they tell me something but I haven’t figured anything out.
 

 
If you want to read the paper - Deformation temperatures of some porcelain glazes – R.T Stull and W. L Howat. Transactions of the American Ceramic Society Volume 16. Page 454. Copyright 1914 by Edward Orton Jr Edward Orton Jr - https://archive.org/details/transactio16amer/page/454/mode/1up

My BFA professor was a sculptor, her work was around 4-5 feet tall and probably averaged an inch thick. She used an Imco sulpture clay, it was pretty groggy. Also fired very slowly. Another thing I've seen is people building reinforcing structure inside the sculpture, extra walls where support is needed. 

Sellling 10k worth of pots is very doable. And if you’re running a business, purchasing equipment is an expense. 

Don’t be afraid to buy clay. You just have to remember it’s a product with ingredients that aren’t 100% uniform over time, and mixed by humans who have off days. It would be odder to not ever experience some kind of weirdness from time to time. If you find something, tell the manufacturer and the retail place you bought it from. They can’t fix a problem they don’t know about, and it’s a reasonable expectation for them to do something to make it right for you.

A lemon test won’t rule anything in as safe, but it will rule out things that are susceptible to acid. That said, alumina isn’t a super volatile material in ceramics, and generally* adds to the durability of a glaze. It’s pretty refractory. That said, if it’s a dry, powdery lump of kiln wash sticking out of the glaze, that’s not good for craftsmanship or aesthetic concerns. You could try glazing over it and re-firing. 
Even if the alumina was not bonded and someone ate out of this piece, you also have to consider whether there’s enough material there to do damage to anyone. I’m not saying you should leave kiln wash bits embedded in your piece. I say that only to contextualize potential risks.
*assuming the rest of your materials are also within reasonable parameters, and the glaze isn’t already an alumina matte, which would be a logical assumption in a liner glaze. 

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. 

Don’t discount the Stroke and Coat. While they’re technically a glaze, it behaves like an underglaze that happens to go glossy. It’s quite stiff, and if you don’t cover it with a suitable clear at earthenware temps, it can show raised brush lines. They’re smooth at cone 6. I have not found them to be translucent at all. I have a friend who runs a paint your own pottery place, and they use them as underglazes, with a coat of clear over top. They don’t run or blur. 
 

Clarification on the Glazy post: the image of that glaze on White Bear clay isn’t Joe’s, it’s another contributor using that glaze. The accreditation is in the gallery. Her name is Michelle Smith. https://glazy.org/u/mlsmithsbcglobalnet/recipes

If you run a bit of 220 grit wet/dry carbide sandpaper (I get mine at Canadian Tire), it should take off stubborn alumina remnants. If you feel fancy, you can tack some to a bat and make a quick and dirty polishing wheel. 

Clarification on the Glazy post: the image of that glaze on White Bear clay isn’t Joe’s, it’s another contributor using that glaze. The accreditation is in the gallery. Her name is Michelle Smith. https://glazy.org/u/mlsmithsbcglobalnet/recipes

Clarification on the Glazy post: the image of that glaze on White Bear clay isn’t Joe’s, it’s another contributor using that glaze. The accreditation is in the gallery. Her name is Michelle Smith. https://glazy.org/u/mlsmithsbcglobalnet/recipes

Thanks. The angle and lighting of the photo was playing tricks on my eyes. I would say that the size of the ring is at least partially to blame. It's adding a lot of mass to the bottom so you have a wide thin platter with a thick area at the bottom. Clay prefers to be evenly thick throughout. There would be far less stress on the piece if the foot ring was much less bulky, like 1/4" to 3/8" thick. Also be sure you're attaching the ring when the slab and ring are at similar moisture level. It's also possible that the platter cracked or was at least stressed at the lip before it ever went into a kiln, and finally opened up in the glaze firing. But because the crack extends all the way into the foot ring it's probably because of the added mass there.

Knowing this, firing hotter should allow the glaze to take the copper into the melt and go green.  I don't know how the cooling will affect it, but I expect if it went glossy green it should stay that way with a fast cooling. The question will be whether or not it will respond the same in a refire as it does in a first firing. Nothing to do but test it!

As Kelly said, options for having absolutely nothing to worry about in a well balanced stable glaze would be either zero colourants or just zircopax or iron.
If you want to use a stain or a colouring oxide and want to ensure it’s safe then lab testing isn’t going to break the bank. It’s about $35 per oxide to get tested and piece of mind.
BSC testing link https://bsclab.com/pottery-testing
Home testing, using an acid such as household vinegar or lemon/lime and dishwasher testing  don’t rule a glaze “in” as being suitable for food surfaces but it will rule glazes out.

PM me if you want a printable copy of the Mason reference guide. ( I converted it to a pdf for my glaze chem class notebook)

You fixed it after bisque, when you fired it again, what cone was that to?
The tension causing the crack doesn’t go away, it’s set up when you’re making the work. Firing exacerbates it. Bisque firing, a little bit, higher firing, a lot. A lot of tension makes a bigger crack. At cone 6 (or whatever your clay matures at) there are massive changes in chemistry and shrinkage. The two big shrinks are wet to bone dry and bisque to glaze, if you’re firing to vitrify the clay. Bisque shrinkage is negligible, but enough to expose hairlines that will greatly expand upon maturity in glaze firing.
Post photos and you’ll likely get a solution to avoiding the crack in the first place. You’re in a community of crack doctors. Or at least a crack perseverant population. 

Babs
Zinc is processed two ways: the French process and the American process. The American process is just simply “roasting” (we call it calcining) that removes impurities. It is denser because it typically 80% pure, with 20% of various minerals commonly associated with zinc. The French process involves vaporing zinc at higher temps, resulting in a purity above 99%. It is finer, with less density. The zinc you are looking for is called Maximo 910 in the States, unless it has been sold to another company yet again.
Tom

I've no experience in this area, but your problems might be to do with allowing time for crystal growth rather than reaching "maturity".
Just a standard reference to slow cooling in an electric kiln, to emphasise sometimes changes in appearance are controlled more by the cooling cycle than by the peak temperature. Which may require down-firing in a kiln that cools too rapidly for that glaze/effect.

Super Cool! Slow Cooling in an Electric Kiln
https://ceramicartsnetwork.org/daily/article/Super-Cool-Slow-Cooling-in-an-Electric-Kiln
 

I rhink you inadvertently did a reduction firing. Easy to do in a small gas kiln.
Do you have time to refire one as a test making sure you have a  fully oxidised atmosphere? With that amount of copper I could get black pots without trying in my gas kiln if not careful re atmosphere.

The vent systems for electric kilns here mostly involve a 10 cm(ish) hole in either the roof or wall, similar to what a clothes dryer uses. Plus ducting, mesh to keep animals out, a suitable fan and a sheet metal hood that funnels fumes either from the top of the kiln or draws them out through the bottom. So they’re very doable in a situation like you’ve described. 

With the caveat that my kiln does not have a digital controller that would be much more subject to temperature variations, yes, I am proof you can easily run a kiln at 0 C. Mine is in a metal shed outside. The kiln does start to struggle a bit at  -30 C, and at that point I can’t blame it. We don’t get those temperatures for more than a week or so in most years, and they tend to come at times where I can easily wait to fire. The power consumption doesn’t go up measurably: 20*C difference in temperature matters to human comfort, but not to a kiln. 
It’s not uncommon where I live for potters have kilns in their garages, which are typically unheated and are often uninsulated. A digitally controlled kiln will need to have a temperature reading from the thermocouple of 0 C before it will turn on, and what many do will put a small space heater inside the kiln to warm the area before they begin loading. Once the kiln is generating its own heat, it’s usually fine.
Ventillation is another matter. Even with an electric kiln, you will want to make sure it’s vented properly. As the different minerals loose organic components, those carbonates and sulfates will need to go somewhere, and that shouldn’t be into your home, never mind where you sleep. In North America they sell kiln vent kits for the purpose, but you should check with whoever sells you the kiln about locally available equivalents. 
The only time I’ve seen gas kilns indoors was in institutional settings, where the building was purpose built and incorporated a LOT of venting. Otherwise they should be outdoors, but sheltered from the elements. They emit everything an electric kiln does, plus combustion, plus presumably some incomplete combustion for reduction.

With the caveat that my kiln does not have a digital controller that would be much more subject to temperature variations, yes, I am proof you can easily run a kiln at 0 C. Mine is in a metal shed outside. The kiln does start to struggle a bit at  -30 C, and at that point I can’t blame it. We don’t get those temperatures for more than a week or so in most years, and they tend to come at times where I can easily wait to fire. The power consumption doesn’t go up measurably: 20*C difference in temperature matters to human comfort, but not to a kiln. 
It’s not uncommon where I live for potters have kilns in their garages, which are typically unheated and are often uninsulated. A digitally controlled kiln will need to have a temperature reading from the thermocouple of 0 C before it will turn on, and what many do will put a small space heater inside the kiln to warm the area before they begin loading. Once the kiln is generating its own heat, it’s usually fine.
Ventillation is another matter. Even with an electric kiln, you will want to make sure it’s vented properly. As the different minerals loose organic components, those carbonates and sulfates will need to go somewhere, and that shouldn’t be into your home, never mind where you sleep. In North America they sell kiln vent kits for the purpose, but you should check with whoever sells you the kiln about locally available equivalents. 
The only time I’ve seen gas kilns indoors was in institutional settings, where the building was purpose built and incorporated a LOT of venting. Otherwise they should be outdoors, but sheltered from the elements. They emit everything an electric kiln does, plus combustion, plus presumably some incomplete combustion for reduction.