Liner glaze options and Hulk's Bowl

[Pyewackette]

@Hulk Just wondering - did you chatter the inside of that bowl?

Also I've been doing a lot of thinking about liner glazes.  I don't want to use any toxic substances AT ALL inside something people are going to be eating out of.  At first I thought I could use Mason stains instead of things like cobalt and chromium and copper, but turns out there's no way to be sure of what they used to make the stains.

But now I'm thinking, after seeing these bowls, maybe in at least some cases (where the clay body is pretty, unlike the old studio clay that turned out the color of zombie skin at anything above bisque) I could just use a clear.  Otherwise I'm good with being limited to white or brown ("safe" colorants) for liner glazes.

Is there any reason a clear glaze couldn't work as a liner glaze?  Do colorants add anything towards cutlery marking and durability?  Anti-crazing?

[neilestrick]

A glossy white glaze is just a clear that's been opacified, so a regular clear is just fine as a liner. Plus a clear is less likely to show cutlery marks than a white. A liner simply needs to be durable and safe, which doesn't necessarily mean it can't have colorants in it.

[Hulk]

Hi Pye,

The marking inside the bowl is red slip, added shortly after trimming - whilst the ware is still damp*.
I'd blunged up some red stoneware clay (with the Studio blender, a thrift shop special!) with water, then ran the slurry through the 80 mesh sieve to remove the sand and other bits. A few cups goes a long way.

I've brushed the slip on the outside of wares, using the wheel, then carved design/pattern after the slip had dried some.

On the inside, I've done some stippling, like the one depicted above, dabbing the slip brush as the ware turns on the wheel. After bisque, any lumpy/rough parts sand away easily. Some of the slip "disappears" in the glaze - the effect is muted, somewhat, by the glaze and glaze firing...

Liner glaze, I'd decided, early on, to stay away from metallic colorants, even though there are stable/safe/tested recipes "out there" from trusted sources. For design underneath liner glaze, I believe the slip is ok, and underglaze should be fine as well, also some small droplets of (stable) colored glaze underneath or on top. 
Commitment to utility! Tough, easy to clean, doesn't mark, un-crazed, well behaved (applies well) glaze as a liner.

My liner is clear-ish. Over red, black, or dark brown clay, the Zircopax - at 2%, a translucent white - shows up.
Over white clay, it appears to be clear (but it ain't!).
The Zircopax to lower COE a bit, add some toughness, and reduce bubbles.
Zircopax (digitalfire.com)
Liner Glaze (digitalfire.com)

*but stiff enough to take on some more moisture - from the slip.

[Min]

2 hours ago, Pyewackette said:

Mason stains instead of things like cobalt and chromium and copper, but turns out there's no way to be sure of what they used to make the stains.

 

Mason puts out a Reference Chart that lists all the ingredients in each of their stains. 

https://www.masoncolor.com/reference-guide

[Pyewackette]

@Min Wow, ask and ye shall receive!  Thanks for that.  Now all I have to do is figure out which ones are unacceptably (to me) toxic in a liner glaze LOL! Turns out not EVERYTHING can be found on Digital Fire.  Just NEARLY everything.

I've also learned what turned a bowl I glazed a weird mixture of purply-brown and gray. A glaze defect known as Boron Blue.  Some people actually use that glaze on purpose to make that.  I think its hideous.  Now if I could only remember which glaze that was ... Probably I can identify it by the list of ingredients on the outside of the bucket.  Now all I have to do is enter the entire list of glazes into the glaze calculator LOL!

[Dick White]

5 hours ago, Min said:

Mason puts out a Reference Chart that lists all the ingredients in each of their stains. 

https://www.masoncolor.com/reference-guide

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)

[Pyewackette]

@Dick White Any chance your notebook includes info on which of those oxides are potentially toxic in small amounts in a glaze?  

[Dick White]

@PyewacketteAll of the oxides in Mason stains are regarded as safe if used in a stable glaze, no different than if said oxides were used as colorants in the typical amounts in a "normal" glaze. The reds and oranges contain cadmium and selenium which would be toxic by themselves, but Mason encapsulates them in zirconium so that they do not leach out of the stain particle. The advantage of using stains to color your glazes is the variety of colors that can be easily achieved without knowing oxide combinations and amounts to add to your base glaze. The disadvantage is cost. As with so many things, you can figure it out yourself the hard way, or you can buy your way out with the gentle application of some folding green.

[Pyewackette]

@Dick White All well and good but there are an awful lot of variables involved in determining whether or not a glaze is stable that won't necessarily show up without lab testing.  Since I can't afford said testing to start with (and given there are many ways for a formerly stable glaze to destabilize, such as material changes, firing variations, and mistakes in mixing it up) I have decided to stick with the safest possible formulations for my liner glazes.

Plus there's the whole expense issue, as you mention.

I'm not particularly concerned about exteriors (its not like I'd use anything leaded) but my liner glazes are another matter.  Probably I don't HAVE to be that careful, but I so choose.  If I assume the worst and just avoid anything that might be problematic in less than ideal circumstances I'll feel more comfortable splurging on the occasional excursion into "safe" colors in a liner.

[Kelly in AK]

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. 

[Pyewackette]

I fully well realize that others think differently on this issue.  However I choose to err on the side of caution.  Hesselberg & Ron and Tony Hansen of Digital Fire both raise the issue of not knowing for sure whether or not your glazes are TRULY durable/"safe" without proper lab testing, lemon tests and dishwasher rides notwithstanding.  I choose to avoid the more toxic colorants in liner glazes, period paragraph.  I would just like to know which ones those are. Thanks.

[Min]

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.

[Pyewackette]

You don't need to test the oxides.  You need to test the GLAZES, each and every one.  Wondering why I can't hold my own opinion on this matter, I'm not telling anyone else what to do or not to do.

As far as I'm concerned it is way easier to stick with a few glazes you know are durable and safe.  A clear, white, or brown (liner) glaze can easily be found that fits my criteria.  Still every once in awhile I'd like to be able to add a splash of color to my liner glaze and still be confident it fits MY criteria.

Nobody else has to do as I do.  However I would appreciate any help offered in identifying the stains that fit MY criteria.  Which mostly just means identifying the oxides I want to avoid in a liner glaze.

EDIT:  @Min Thanks for the testing link.  Bookmarked.  I may want it someday.

Edited November 27, 2023 by Pyewackette

[Min]

25 minutes ago, Pyewackette said:

You don't need to test the oxides.  You need to test the GLAZES, each and every one

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. 

 

[Kelly in AK]

@Pyewackette I looked back at my post, it was a late night just-before-bed ramble and did go on and on a bit. I think I may have missed making a clear point in all of it. I didn’t mean to come across as dismissive and apologize if that was my tone.

I can’t think of a way to differentiate the dangerous coloring oxides from the safe ones. They’re all very specific in their risks. For example, I think of cobalt as risky because it’s so damn hard to see until it’s high fired. I could have it on my fingers without a clue while eating fried chicken. Two percent or five percent in a glaze bucket looks about the same. Iron’s not like that. You absolutely see it.  The two I can think of that pose virtually no risk are iron and zircon.

The only way I understand to mitigate all chances of toxicity is either to lock the colorants in a glass that will not degrade under the most extreme conditions or put them underneath a layer of that glass. If you put too much of a colorant in glaze it’s not the metallic oxide that makes things dangerous per se,  it’s that you weakened the glass enough to allow it to degrade and leach that metal. Encapsulated stains are arguably less likely to leach their coloring oxides, but thankfully their ingredients are published and you can choose the way you use them. There was a time (Not so long ago!) when leaded frits were argued to be safe beyond reproach. That’s just to say, I respect where you’re coming from. 

To go further, barium and lithium aren’t coloring oxides, but used as fluxes and adjuncts to color, yet have known toxic properties. But there I go again. Too many commas in one sentence. I gotta go to sleep! Nailing down what’s dangerous is tricky, I believe we all want to do that. It’s a journey. Stay away from lead. 

[oldlady]

pye, min gave me a perfect glaze recipe for the one clay i use.  as a clear it is totally clear, no bubbles.  thank you min!!!    because i am making work that relies on color on heavy relief,  i use that clear recipe and add various mason stain colorants to the glaze recipe.  i want to see  transparant color in the deeper areas of the relief and on the parts above.  i will have some photos soon that i think i can post here so you can see what i mean.  the percent of color is usually under 4.  cobalt is so strong it is 1%.

i have a perfect very white liner glaze that i have used for many years.  i can send you one bowl that has been in daily use since 2000 .   the recipe was from a book written by a ceramic engineer in 1972 and there has not been an update with warnings since.  so i think it is safe to continue using it.  

i trust the experts but keep my eye open when i use a piece to see if it holds up under normal use.   there is no excuse for selling something dangerous but the likelyhood of that happening is so remote when the source recipe and the stains in use are well researched by the developer of the glaze.  i have never seen a headline about DANGEROUS POTTERY!  AVOID BUYING !  since the announcements about lead in glaze 40 or 50 years ago.  (i bet you can't find lead to use in glaze unless you leave the usa.)

i think you are going  just a little overboard in worrying about this.  no, i do not have a list of dangerous (???) additions to a base recipe.   i don't think many potters do.

[C.Banks]

On 11/26/2023 at 7:45 AM, Pyewackette said:

 

 I've been doing a lot of thinking about liner glazes.

 

You are not alone here.

At most I'll risk a dip on the rim or pouring swipe of some safe matte for slight changes in texture mostly.

There are many more than two of us keeping metal oxides out of our liner glazes I imagine. The iron in tenmoku is an exception here.  A case can be made for copper too except for the one medical condition - so copper is out.

Opacifiers like tin and titanium are ok so this helps us find more shades of iron. Zircopax can help improve durability apparently and if I remember correctly magnesium helps with glaze fit?

I know 'good glass' will prevent leaching but the cynic in me sees more people concerned with projects other than a sound liner glaze - this includes craze free clears.

As liners, clears are the most difficult in this regard, imo. We don't get to hide behind tenmokus. And even though a properly vitrified clay will forgive some crazing its easier to simply promote a crackle aesthetic than develop and maintain a craze free, clear surface.

My pots might tend towards boring but idc too much. I do tend to overthink things so keeping things simple can help - sometimes.

 

 

[High Bridge Pottery]

I was talking to a guy at work who did some glaze leach testing with lead glazes and grapefruit juice. They were getting really high lead readings but it turned out to be the grapefruits being grown near a road and nothing to do with the lead glaze. As safe as we can make a glaze a lot of the food/drink we put in is contaminated