dhPotter

I prefer to use the term "durable" rather than food safe when talking about glazes. There isn't one agreed upon definition of what makes a glaze "food safe". In North America if a glaze does not contain lead or cadmium it can be labelled food safe. This is a very wide definition and adding something along the lines of what Mayco says "Finishedware producers bear responsibility for food safe testing their ware being sold into commerce." IMHO isn't good enough. Like Callie said Strontium Crystal Magic (SCM) is  fine on the outside of pots where your mouth or food won't be in contact with it.
Re lithium and toxicity, just adding a little bit. Topic of lithium and toxicity has come up in many discussions in many forums etc over the years. My impression of the reading I've done is it might not be an issue for most people but for those on lithium therapy it can be, both in the process of mixing glazes and leaching glazes.  Poison is in the dosage scenario.
With all due respect to Callie I found SCM quite rough, dry and unmelted. You could try it by itself on a test tile and see how you find it but it's a bit of a moot point because it isn't used by itself.  The overload of titanium is what makes the crystals/variegation when layered with other glazes. A good way to get interesting glaze reactions is to layer 2 or more dissimilar glazes, what one glaze lacks or has a surplus of reacts with the other glaze, this often results in interesting visual texture. SCM is loaded with fluxes to react with other glazes, that's part of the reason why it works.
Re silica and glaze durability, having as much silica and alumina in a glaze is one of the ways to ensure a durable glaze.  For cone 6 having at least 2.5 molar of silica is a good starting place, if the glaze can dissolve more then it's a good idea to add it if you are looking for a durable glaze. If you look at the screenshot above in the bottom half of the image you can see the silica is 1.68 so at the bare minimum it is about 1/3 short of where it should be. Overloading a matte glaze can quite often turn it into a gloss glaze  and really oversupplying it can turn it back into a matte and crazed glaze so there are parameters within which to work. 
Another aspect of silica that is often overlooked is the mesh size of the silica being used. When you mix glazes using a 325 mesh size silica more of the silica will likely be dissolved in the glaze than if you use 200 mesh. (not applicable for the SCM)
One more thing is to look at the colouring oxides used. Don't use crazy amounts of colouring/transition oxides, use the least you can to still achieve the effect/colour you are after.
There is a good article on toxicity that includes materials here if it helps.

Toxicity is a whole entire subject by itself. Mostly we have to start with definitions to help clarify. 
Tl;dr, yes you can put this glaze on the outside of a pot and it will be fine. It’s not touching food. It melts fine at cone 6, it’s just a “soft” glaze.
When a raw material is labelled toxic, that means it’s toxic in its current form. Once you put a material into a glaze with other materials and fire it, it becomes another material entirely, with different properties. If you’re using a commericallly mixed glaze that is labelled non-toxic, that means it’s not toxic in the unfired state. (It’s probably not toxic in the fired state either, but you’d have to test it to confirm, and manufacturers can’t say that it is for sure under all circumstances without legal problems. People do weird and unexpected things.)
When we’re talking about glazes, most often the finished glaze is more safe than, say, raw lithium. So potters should be concerned about their own health and safety first, and the end user second. Materials like lithium can be handled safely with sensible studio hygiene practices, and can be beneficial in a lot of good glazes.
When knowledgeable glaze chemists talk about food safety, we aren’t talking about whether a glaze will leach Bad Things (TM) into food. This is a very infrequent scenario outside of lead use, and lead hasn’t been used in most handmade pottery for decades now. But we are thinking about craftsmanship and durability. We’re asking questions like Does this glaze have flaws like crazing or pinholing, that can affect structural integrity or possibly harbour bacteria? Does this glaze scratch, or does it cutlery mark? Does it stain easily? Is it easy to clean? If those problems are solved, leaching is usually a non-issue. 
Re the silica question: You need 3 types of components in the right proportions in a base glaze for durability: silica to form glass, alumina for strength and to make it not run too badly, and different metals that make silica and alumina melt at much lower temperatures than they do by themselves (fluxes). Without enough silica or alumina, or if those things are in the wrong proportions, the glaze may react to acids found in many drinks, bases like in dishwasher detergent, or may scratch or cutlery mark. SCM doesn’t supply enough silica for durability, but the strontium is also a problem.
Within the flux category, there’s a bunch of stuff that’s a good idea to use in combination, because each material has beneficial and undesirable properties, and mixing them can bring out the best (or sometimes worst) of both worlds. For example, sodium is a strong flux, which is needed to bring down the melting temperature of silica and alumina, and is in most glazes. However, it tends to make glazes craze when by itself. But if you mix it with calcium (or barium or strontium or lithium and a few others), you can still get the melting, but less crazing.
The calcium/magnesium/strontium etc group of fluxes also give you different colour responses, or can help create matte effects if the proportions are high enough. One of the drawbacks of using strontium or barium though is that neither is very high on the Moh hardness scale, so if you use too much of it, the resulting glaze can be prone to scratching, staining or other flaws. But it helps create that really cool matte surface because it promotes crystallization, so sometimes creative folks make trade offs. 

First off have a look at the materials and their amounts in the recipe itself. I re-totalled the recipe from the original 60.60 to 100 as that is the standard base amount. When we apply a standard amount it makes it easier to notice possible areas of concern. Top part of the screenshot below is the Glazy recipe re-totalled to 100.
First thing that jumps out is there is no silica added to the recipe. Yes, there will be silica in the formula from the feldspar etc but probably not enough. (more on this later) Second thing that jumps out is the amount of titanium in the recipe, nearly 12%, this is a huge amount! Third thing, is approx 40% feldspar for a ^6 glaze is getting quite high. Put these things together and you have a recipe that won't be durable (low silica), probably won't melt properly at cone 6 and is overloaded with titanium.
I know this glaze and the "warm" version of it are very popular at cone 6. It's important to note also that it isn't used as a standalone glaze, it is used as a layering glaze with one or more other glazes. What the combined chemistry is of the layered glaze is what needs to be looked at, will those be durable, I don't know, each combination would need to be looked at.
BTW, giving credit where credit is due, this glaze started off as a Tom and Elaine Coleman ^10 glaze called Yellow Crystal Matte. Barium was removed from original glaze and strontium subbed for it, the gerstley borate was subbed with the Ferro 3124 and the zinc removed.
 

 

I'm seeing a lot of plates made like this these days, it's a good way to make a plate with this type of design. I've got one bisque fired as a test but haven't got it glazed yet, it was really fast to make and I also like that it doesn't require any trimming. That nearly vertical edge is going to help prevent sagging/warping during the glaze fire compared to a flatter more horizontal rim.
I would suggest putting one of yours in a glaze firing with no glaze on it, if it doesn't warp then it would suggest the glaze fit could be the problem.
Welcome to the Forum.

I use 4 main glaze colours for dipping glazes, all made from one base plus a clear liner, these are in 5 or 6 gallon buckets plus an accent glaze that is just a small container of glaze. When I used to spray glazes I had more glazes but could mix up smaller amounts of them since I didn't have to have a big bucket to dip them in. I find that using less colours makes a more cohesive looking display, I don't try and please everyone by offering every colour under the sun.

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. 

That type of crack is likely to continue to get worse, especially in the glaze firing. Sometimes it's best to just move on and make another one.

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 

Focus and Follow-through!!
Planning ahead and dealing with the unexpected.
Learning to criticize constructively.
Learning to accept criticism constructively.  
Asking “Why” and “How did this happen?”
 
 

Throwing out another alternative, the Hyde Stir Whip. I had problems with my Jiffy mixer scraping plastic from the sides of the buckets over time so I started using a Stir Whip. I've  it for about 9 months, so far so good. I use an electric drill, haven't had splashing problems. It takes a few seconds longer to get the mass of glaze moving but once it is moving it mixes really well. 
 
 

@Roberta12 This mixing matte and gloss works great!! I have a base of G2934 at 40% and a clear, G1216M, at 60%. This base has given me white, tan, yellow, turquoise and gray liner glazes. The glazes are very durable. 

@grackle, for dark clays or those with a lot of impurities to burn out it's important to slow the bisque firing down between 1300F through to 1650F. It's in this temperature range inorganic carbons will burn out. Try slowing your bisque firing down 60 -100 degrees F per hour and see if that helps. For dark clays coloured with manganese try firing to only cone 5.
Another way to get a nice matte glaze and be able to adjust how matte you want it is to blend a gloss glaze (without colourants or opacifiers) with a quite matte glaze. If you do a simple line blend with the gloss and add increments of 10% of a matte glaze you can see how adding the matte will negate some of the gloss. If you want to try this a good recipe to use is this one. Once you have the level of mattness (if that's a word) you like then add zircopax (approx 10%) or tin (approx 5%). If you find the zircopax makes it too matte (because it adds silica also) then tweak the gloss slightly higher.

Hi Marie and welcome to the forum!
My experience with the 5% soda ash test when testing the black glaze Licorice from the book Mastering Cones 6 Glazes by Hesselberth and  Roy  led to a degradation of the glaze gloss level. I then went on to leave a test piece in the dishwasher for several months. Real world dishwasher tested did not show any degradation. I do think the soda ash test is too harsh a measure. Note it is within a hair from the "ideal" flux ratio. I did contact Roy about this and he didn't recall doing the soda ash test with this glaze.
If your glaze can take it increasing the silica and alumina in a formula will improve durability  regardless of flux ratios.
Regarding the "ideal" flux ratio of R2O: RO of 0.30:0.70, have a read of this snippet from Matt Katz's 2016 article Glossed Over: Durable Glazes from NCECA 2016
note the test for this article with a 0.1:0.9 ratio was "surprisingly robust" and the need for further research regarding the effects of silica:alumina ratio and the use of colourants. I don't think we are at an absolute conclusion yet in regards to flux ratios. They do not stand alone insofar as durability measures.

You want to look for a stiff glaze (little glaze mobility / fluidity) if you don't want it to break on edges. Think of majolica type glazes, they are high in alumina making them a stiff non fluid glaze so when the overglaze brushwork is put on the designs don't run or move. 
If you don't want to use talc then try your hand with a glaze calculator tool, like the free one on Glazy, and have a go at removing the talc and supplying the now lost magnesium with dolomite  instead. You will have to tinker with the amounts as it isn't a 1:1 swap then will need to rebalance the remaining oxides in the formula. 
I had a look at Tom's Lakeside Blue recipe above, couple thoughts about it. First thing that jumps out at me is the gum in it, I'm guessing it's there because the glaze might have a tendency to crawl without it due to the super high amount of epk. (Epk shrinks as it dries, excessive shrinkage = cracking which more often than not results in glaze crawling during the glaze fire). Bit of a band-aide solution to add gum when supplying part of the epk through calcined epk is simpler. Other thing is the colourants are included in the total, not as an addition. Makes it easier to adjust a recipe if the colourants are a separate addition from the main recipe. If possible I would reduce the cobalt, using less of a transition metal  is preferable if the colour is still okay.
I redid the original recipe first with just replacing the talc with dolomite to supply the magnesium. Then I took that recipe and replaced about half the epk with calcined epk. Glaze with the calcined epk is going to appear thinner in the bucket with the same amount of water. (measure specific gravity) I didn't include the gum in the recipe as it doesn't contribute to the fired chem. If you try this please just mix up a small test amount first as what looks good on paper doesn't always translate to a pot. 

To me it looks as if glazes or slip are layered onto a textured piece and then partially removed by sanding or other method then fired. Almost looks like "raindrop damascus " in a way.

I do this with a bisqued piece because it shrinks just enough to fit inside the cookie cutter but loose enough to not get stuck in the cookie cutter. The bisqued circle gives you coverage over the whole shape, so you are not pushing to release in just a few spots. 

I do this with a bisqued piece because it shrinks just enough to fit inside the cookie cutter but loose enough to not get stuck in the cookie cutter. The bisqued circle gives you coverage over the whole shape, so you are not pushing to release in just a few spots. 

I do this with a bisqued piece because it shrinks just enough to fit inside the cookie cutter but loose enough to not get stuck in the cookie cutter. The bisqued circle gives you coverage over the whole shape, so you are not pushing to release in just a few spots. 

If you use a claybody made for tiles it will be far less plastic than your typical claybody, less plasticity in a claybody helps tiles stay flat. (along with the practices you are already doing and @akilpots suggestions) Also, if you lay a piece of very thin plastic sheeting/film over the clay before pressing the cookie cutter down it will round over the edges so you don't need to smooth the topside eges of the coasters. 

Yes, you can use the L&L tubes, you just have to drill out the TC hole. Also, the tubes have a flange on the outside, so L&L uses little ceramic spacers behind the TC block (screws go through them). You need two for each screw, so 4 total. You can see what I'm talking about in THIS VIDEO. Technically you can do it without the spacers, but things snug up better with them.

I think that Tony Hansen's demonstration of the fallibility of ChatGPT is worth posting.
ChatGPT is completely wrong about the cause of glaze crazing!
https://digitalfire.com/picture/3159
... a fine example of ignoring the elephant in the room.
... a fine example of ignoring the elephant in the room by ChatGPT
PS It reminded me of a very old software engineering joke (1970s?).
A man was been taken on a cross-country flight in a helicopter. During the flight the ground became completely covered in dense fog, and the pilot became lost. The eventually came across the top of a office tower sticking above the fog, and they hovered nearby. The man attracted the attention of somebody in the office and held up a placard saying "where are we?", and received the answer "in a helicopter hovering 60ft above the ground".

The pilot said "OK I know exactly where we are, and set off towards their destination.
Later he explained "the answer you got was technically correct and not the slightest help, so obviously we were at the Microsoft help centre whose location I know".
Maybe ChatGPT is another help centre.

Re wax resist, the type you use overtop of another glaze makes a big difference in how it behaves.
Both oil based and water based wax resist will work but the water based one is much more time and moisture sensitive than the oil based one. If you use a water based wax resist overtop of glaze it can curl up and loosen it's hold on the base glaze. The longer you leave it before glazing over the worse it is.
With wax based resist this issue is avoided.
Some resist labels don't tell you if they are oil based or not though, you can tell by how the resist washes out of brushes. If the brushes stay a bit gummy when washed with hot water then it's oil based. If the brushes easily wash clean then it's water based.

Brushed underglaze and brushed glaze using tape mask

Transparent red glaze under transparent blue glaze, left.
Royal Blue Speedball under transparent red glaze, right.
Pulling the tape where the tape's edge makes a clean cut, as Bill mentioned, helps, also, peeling the tape when the brushed material still has some moisture in it (little bit!) can help, in case the build up is prone to cracking at all. Also much less dust when there's still some moisture in there...
Here, the tape and brushed underglaze was done after bisque fire.
It helps to have a clean surface - no dust - and smooth is better; burnished clay can make for a clean line.
Put the tape down and press the edge firmly - lock it down with your fingernail*.
This was cut strips of plain masking tape (decent tape, not bargain material - 3M masking tape).
Waiting for the stenciled brushed on part to fully dry before glazing makes for a fuller coating.
The liner glaze was poured in, poured out.
The outside glazes were dipped.
I get very little "bleeding" under the tape's edge. If any, I re-cut the edge with a razor knife and then scrape away the bled under part.
I'm getting a smooth all melted together finish with this type of taped off and brushed decoration.

Where spraying is nice - the material hardly ever (ever!) runs under the tape.
Also, it can be easy to get a nice smooth even coat.
"Can be" as there's technique involved...
Some underglazes will move/drift/blur with some glazes - test!
The liner glaze transition was done by Tony Hansen's method (how to liner glaze a mug).
Note the red edge is fuzzier than the blue. It does that.
 
*If you've fingernail to spare, else the smooth rounded part of a loop tool, edge of a rib, edge of a confectioner's spatula...

I bought into the GB not going to be around back in 2008. Have been using Boraq, 1 for 1, as my Gerstley Borate substitute. It was around $70 per 50 lb bag about 5-6 years ago.

This is what worked for me. I use 12 to 17 minutes based on how dense the load is. I also use an offset like Pres mentioned.