Sandy Feeling Glaze

[dhPotter]

It has been since May 2015 that I have glazed anything. I scrapped the buckets, stirred with the drill paint stirrer, and sieved 2 times. All glazes were fine except this one glaze, Floating Red. After sieving, there was a huge lump of material left in the sieve. It had a consistency like wet sand. It would not force thru the #80 screen. I just dump the whole mess into the bucket. When I first made up the glaze all material sieved correctly. When using this glaze I constantly must stir to keep the material in solution.

 

Below is the recipe. What is in this recipe that would combine and make the sand like material?

 

  Boraq.......................                37.80  
  Ball Clay...................                 0.70  
  Magnesium Carbonate.........     8.40  
  Silica......................                  30.10  
  Whiting.....................                 6.00  
  Iron Oxide Red..............          17.00  
                                         =========
                                                100.00

          CaO       0.60*   14.13   17.06
          MgO       0.33*    5.47    9.20
          K2O       0.01*    0.28    0.20
          Na2O      0.06*    1.66    1.81
          TiO2      0.00     0.01    0.01
          Al2O3     0.04     1.50    1.00
          B2O3      0.47    13.80   13.43
          SiO2      1.73    43.38   48.91
          Fe2O3     0.30    19.77    8.37

                        Cost:        
              Calculated LOI:   17.91
                 Imposed LOI:        
                       Si:Al:   49.05
                      SiB:Al:   62.52
           Thermal Expansion:    7.48
              Formula Weight:  239.61

[Min]

I would say some of the boron from the Boraq has precipitated out of solution. If you scrape up all the gritty bits and pour boiling water on them they should redissolve (use some of the water from the top of the bucket so you don't dilute your glaze too much). I know fritted boron is not supposed to be water soluble but I believe it is slightly so. Third picture down on this link http://digitalfire.com/4sight/material/ferro_frit_3195_356.html

 

Are you sure there isn't supposed to be more than 0.70 ball clay? 

 

This glaze isn't for the inside of functional pots right? (or to go in the dishwasher?)

[dhPotter]

Min;

 

Wow. Yes we use bowls that have some of his glaze on the inside. I use them regularly, daily, put in microwave, dishwasher. What are the dangers of this glaze and what is the cause of the danger? This glaze, put on the inside of functional pots, has not gone out to the public. Just in my own use. 

 

When it comes to formulating glaze I'm poking and hoping in the dark.

 

Will definitely try the boiling water trick. Thank You.

 

About 6 years ago, I found this recipe on DigitalFire. I worked it over to use Boraq instead of GB and try to get as close to the original numbers. Is this recipe safe? if not, where did I go wrong in the new formulation?

 

  CaO.........................     11.55  
  MgO.........................       6.51  
  Na2O........................      2.25  
  Al2O3.......................      0.41  
  B2O3........................    15.17  
  SiO2........................     43.43  
  Fe2O3.......................    20.67  
                              =========
                                     100.00

          CaO       0.51*   11.55   13.93
          MgO       0.40*    6.51   10.93
          Na2O      0.09*    2.25    2.46
          Al2O3     0.01     0.41    0.27
          B2O3      0.54    15.17   14.75
          SiO2      1.79    43.43   48.91
          Fe2O3     0.32    20.67    8.74

                        Cost:    0.07
              Calculated LOI:        
                 Imposed LOI:        
                       Si:Al:  179.00
                      SiB:Al:  233.00
           Thermal Expansion:    7.35
              Formula Weight:  247.69

[High Bridge Pottery]

Low alumina, high boron make for a glass that is not very durable I think and probably weak to acid/base. Still I mostly ignore the limit formula now and test if my glaze is durable or not. There are a lot of other factors that come in to play than what numbers you have, but they are good to keep in the back of the mind.

 

I bet the alumina is low for a reason, but I can't remember what low alumina does. Do you get some nice iron crystals? Some high boron variegation? 

 

Not sure about the sandy stuff at the bottom of the bucket. No way it could be transfer from pottery to glaze in the glazing process? My initial thoughts were rutile but there is none 

[dhPotter]

Joel,

 

From L to R -

Floating red on the bottom with my version of Floating Blue overlapped creating the yellow flame licks.

Floating red on bottom overlapped with floating blue in the middle.

Floating red over Licorice (from mastering ^6) - this ran like an Olympic 100 meter runner. Now know not to put matte over glossy.

Floating red over Waterfall Brown (from mastering ^6) - another big runner

Floating red over floating blue; floating blue over floating red, floating red by itself, floating blue by itself

 

 

[Min]

Yay, I’m glad you are using a glaze calc program : )

 

Okay, so have a look at the alumina level to start with. Looking at the unity figures (from the first column of numbers in your recipe) it is 0 point 04. You don’t have to go any further than that to see this glaze isn’t going to be durable. Aiming for 0 point 2 as a guideline lower limit, higher is better up to about 0 point 5)

 

Yes, as HighBridge said limit formulas are conservative guidelines for a non leaching durable glaze but 0 point 04 is way way below the amount that you need.

 

Now have a look at the silica level in your recipe, 1 point 73. This is low but not as extremely undersupplied as the alumina is.

 

I’m guessing this is a very fluid and variegated glaze? If so then I would guess it’s partly from the high boron. (0 point 47) 

 

^6 Iron reds are tricky, they are usually low in silica and or alumina. If you are looking for an iron red that is more stable I have had success with Bailey’s red and Van Gilder’s Crocus Martis Red.

 

For that second formula you wrote, have a look at the alumina…. Nuff said.

 

Re: “food safety†of this glaze, unless a person has unusually high blood iron levels I don’t think there is too much concern with the iron leaching out but no way is it durable. To see what I’m talking about take 2 test pieces that have been glazed. Put one in a stainless pot with 50 grams of soda ash and 1 litre of water, bring it to the boil then simmer it for 6 hours (with lid on). Rinse it off, dry it and look for differences between the boiled test piece and the unboiled one. 

[dhPotter]

   

 

This is what I was trying to get.

 

http://digitalfire.com/gerstleyborate/recipes/ironreds.html

 

          CaO       0.51*  
          MgO       0.40*   
          Na2O      0.09*  
          Al2O3     0.01 
          B2O3      0.54  
          SiO2      1.79  
          Fe2O3     0.32 

 

I found what I originally started out with on DigitalFire. I backed into the recipe using materials to match up the formula numbers.

 

Can you help me with trying to understand where I went wrong? Are there better materials to use? Maybe a Frit?

 

Thank you soooooo much!!!

[Min]

To get that lovely glaze Hansen has the alumina at nearly zero. It's gorgeous but as it stands it won't be durable. I just did a quick read of the link, Hansen seems to answer to your question at the end of the article.

 

"Use a less fluxed and therefore less fluid underbase version of the glaze and a thin application of the fluid overtop. How? Use INSIGHT to increase the silica/alumina in the iron red glaze while maintaining their ratio, apply this less melting version in a thick layer, then apply a thin layer of the original glaze. The result will be the visual character of the thicker version without the runniness. Of course, this means you need to have two glazes and double dip, but it is worth it for iron reds. For this to work your glaze cannot have excessive shrinkage during drying or it will crack and peel after double dip. Here is an interesting parallel: we found that Randy's Red (15% iron) gives the brighter color associated with a thick layer and yet does not run when applied in a thin layer over Ravenscrag slip on porcelain and whiteware bodies."

 

I don't think you did anything wrong in coming up with your recipe, it's the formula you are basing it on that is the problem. I don't think Hansen meant it to be for functional pots. He's a lovely man, send him an email with your questions and see what he says.

 

Sorry, gotta run, I've a sale starting in a few hours, hopefully someone else can chime in here.

 

 

 

[dhPotter]

Min,

You have been and always are a wonderful source of information.

 

Thank You

[High Bridge Pottery]

Looks like a pretty good go at the glaze but the surface looks that way because the glaze is 'wrong' Low alumina high boron give it that lovely look. It's interesting in the article that somebody who has tried many of these glazes has gone on to use a lead, I presume functional, red glaze which comes with similar firing difficult but they say is much more consistent in its look. They also seem to have used these kind of low alumina reds for years before the change.

 

"Here is a comment from Paul Lewing: "I've tested and used dozens of versions of iron red glazes ... They are all runny and very sensitive to temperature and even more to rate of cooling, and are very affected by the brand of iron you use. And they won't be red if the Si/Al ratio is under about 18:1, or if there's more Mg and Ca. Food-safe? maybe. Durable? no way. I've given up on them in favor of a lead glaze I call Drop Dead Red, which has all the same post-firing problems but does at least come out consistently iron red and glossy.""

 

There are a couple of glazes there too that have more alumina and silica which they say are red but no pictures. It's all a question of how durable you need the glaze to be, is being used once or twice a day heavy use? I am not quite sure of the answer, never done much testing.

[Pieter Mostert]

John Post has a nice cone 6 iron red that's within the limits for Al2O3 and SiO2 (depending on which limits you use. I agree with High Bridge about limits just being a guide). It's more orange than yours though. Have a look at the notes on John Post's website  - it seems that if you want to try this glaze, you'll probably have to do some fine-tuning.

And while you're at it, check out his Currie grids.

[Celia UK]

Going back to your original post dhPotter - I had exactly the same thing happen earlier this year.

It was a Low fire glaze mixed at a glazing workshop -

Calcium borate frit 39

Potash feldspar 27

Quartz 23

China clay 6

Whiting 5

+ Copper oxide 2

Fired to a strong, shiny turquoise. Months later - solid, sand-like layer at the base, just as you describe above.

 

An experienced potter in my local group did explain what had probably happened but said the change was irreversible - I dumped the sand-like crystals and kept the remaining 'glaze'. Have just been to look and see what happened when I used it as a test, but can't locate the result. In my memory it still fired to a glossy finish, but nothing like the original turquoise. Am wishing I'd kept the seivings and tried dissolving in hot water!

 

Interestingly, I mixed another v small sample but replaced the Calcium Borate Frit with Standard BF and the Potash Feldspar with Neph Sy as I had these to hand - ( no CuCo2 added) and this solution has not crystallised out, in the way described above, but solid matter settled heavily to bottom (stirs up ok) Not tested yet.

 

I'm intrigued once the chemists among you start posting your thoughts - it just reinforces in me my ignorance and realisation that I can only ever understand this at a functional level, which means more trial and error than scientific testing! I hold my hat off to all the 'experts' out there.

[PeterH]

Celia: The best discussion I've seen on this topic is:

http://www.ceramicstoday.com/articles/flambe_magic.htm

 

PS Should have said: Doesn't alter the wisdom of Min's boiling water advice,

but elaborates why it is a good idea.

[Celia UK]

Thanks Peter - interesting read. Now I REALLY wish I'd kept the crystalline bits!

[dhPotter]

PeterH,

Great article. Describes what was left in the sieve from the Floating Red.

 

"Soluble and slightly soluble compounds include borax, soda ash, pearl ash, Epsom salts, wood ash, gerstley borate (and its substitutes), copper sulphate, nepheline syenite, lithium carbonate, barium carbonate, etc."

 

Would the "etc" also include magnesium carbonate?

 

Pieter Mostert,

Currie grids have some real nice glazes to test. I have seen John Post's glaze before but shied away from the bone ash.

 

[Pieter Mostert]

dhPotter, I'm curious why you want to avoid bone ash. I've been planning to buy some to try in a few glazes, so it would be good to know if it creates any problems.

[dhPotter]

Pieter,

 

Seems I have read that the bone ash will make the glaze slurry stinky and the slurry will gel in a short time. I'm not glazing very often, yet, so do not want to waste glaze material that will need to be thrown out.

 

If I am wrong in my assumptions please correct. I have seen some glazes I wanted to try but moved away from them because of the bone ash.

[Min]

You can sub tricalcium phosphate 1:1 for bone ash.

[PeterH]

Seems I have read that the bone ash will make the glaze slurry stinky and the slurry will gel in a short time.

 

AFAIK the stinky problem only occurred with uncalcined bone ash, but tricalcium phosphate must be a more consistent product anyway.

 

As to gelling, from http://digitalfire.com/4sight/material/bone_ash_123.html

Bone ash is not common in glazes. When employed it can cause the slurry to flocculate and thicken (and produce a very thick layer

on the ware surface which cracks during drying). People often react to this by adding more water, producing a glaze that shrinks even

more on drying and eventually thickens again. A better way is to add a little deflocculant to the glaze slurry (like Darvan).

 

 

 

 

 

 

 

[Tim T]

I had a glaze I used a while back that grew very large crystals. Quickest fix I found was to nip into the kitchen whilst my other half was out and bung it in the microwave, and the crystals dissolved into the hotter water.

[Pieter Mostert]

 

"Soluble and slightly soluble compounds include borax, soda ash, pearl ash, Epsom salts, wood ash, gerstley borate (and its substitutes), copper sulphate, nepheline syenite, lithium carbonate, barium carbonate, etc."

 

Would the "etc" also include magnesium carbonate?

 

 

 

According to Hamer and Hamer, magnesium carbonate is slightly soluble in acidic water, although the light form is less so (digitalfire calls it practically insoluble). I think the Boraq is a much more likely suspect though.

 

A while ago I mixed some magnesium carbonate with vinegar, just to see what would happen. There wasn't any noticeable reaction, like there is with calcium carbonate, but after the vinegar evaporated, what was left was a gooey paste (magnesium acetate?) that never dried out. When I did the same experiment with calcium carbonate / Whiting and vinegar, it dried out to form a combination of crystals and little gnarled structures like you see in cave formations.

 

So perhaps Whiting can give rise to little granules if the glaze is acidic enough. In this thread calcium gets blamed for a similar problem to yours, but so does Gerstley Borate.