SCM at cone 6. Glaze Issues, Firing Temp, and Chemistry Questions

[Joseph Fireborn]

This has been created to carry over the conversation that we were having on the discussion comments instead of in a topic. I have copied and pasted the discussion that I created in order of start to current. Please lets move all topics here as it would better be searchable in the future. 

Comment by Joseph Fireborn 

I have a question about SH's pots. I have tried using SCM, I get some really nice results, but the glaze surface always feels so gritty and sandy. It isn't really sandy or gritty but the feeling is rather odd. Does anything else get anything like this or I am doing something wrong?

Comment by Norm Stuart 

My impression has been SCM requires more than a Cone 6 to achieve the results I see others showing. The same too with Jen's Juicy Fruit. For me at Cone 6 both have too much unmelted materials - particularly the strontium in SCM.

Comment by Joseph Fireborn 

So you think I should try getting closer to cone 7? I can do a hold at cone 6 and hold for like 30mins or something and see what happens.

Comment by Norm Stuart 

Catherine Rehbein fires a lot of clearly glossy melted strontium crystal magic pieces, so I'd be very curious what firing she uses to achieve this.https://cone6pots.ning.com/profile/CatherineRehbein

Strontium oxide produces brilliant colors like lead with low thermal expansion and can be a flux above Cone 2.  But Strontium Carbonate doesn't decompose until an astounding 2,721 F, a good 350 F hotter than Cone 12.

If you're interested in an experiment, before adding the strontium carbonate to the glaze, I'd first react it with an acid like vinegar or hydrochloric acid to release the carbon dioxide.

Strontium acetate and strontium chloride both decompose below Cone 010, but at the expense of being far more soluble than strontium carbonate. I'm curious if this would make any change in the firing behavior of SCM.  It seems unlikely but potential surprises lie in wait out there.

Comment by Joseph Fireborn 

So soaking strontium carbonate doesn't sound that hard. Do you think it would fully absorb into the vinegar? Or do you think if I evaporated the vinegar it will return to a type of powder?

I guess I will have to test and see. Im thinking I will mix a 2 test batchs of SCM, and a control batch. I will put the stront carb into the vinegar and let it soak overnight. I will then just directly pour one into the glaze mixture and then adjust flocculation with darvans. Then I will take another vinegar batch and try to dry it out and see what happens. If I can dry it out I will run another test using that SCM batch, and I will run a control batch just to compare. I will run the test using currie grid to see if there are some interesting results that I might be missing. 

It will be a few days before I can run the grid test, I grid tiles drying right now, but how long do you think I should soak the SCM? 24 hours 36 hours? 48 hours?

I have a friend who does a lot of experimental stuff with glazes I will contact him and see if he has any opinion. 

Catherine Rehbein's work is really nice and colorful, and your right it is glossy and not a sandy matte look like SH's work.

Comment by Norm Stuart

Acids combine with stronitum carbonate to bubble off the carbon dioxide (the carbonate). What you end up with depends on the type of acid used.

Vinegar (acetic acid) creates strontium acetate.  Nitric acid would create strontium nitrate, sulfuric acid strontium sulfate etc.

If Vinegar is strong enough to get the reaction with strontium carbonate going you'll immediately see foaming as you add the vinegar to the strontium carbonate and the carbon dioxide and water vapor bubble off. It should look just it does when you add vinegar to baking soda.

If vinegar is not strong enough to start the chemical reaction on its own, you'll need to heat the mixture over a stove, use a stronger concentration of acetic acid, or more realistically use a stronger acid like hydrochloric acid (called Muriatic Acid at a pool supply store) packing enough energy to break off the CO3 as CO2 and water vapor.

As it sounds like you're new to chemical reactions, always add a strong acid or strong base to the chemical to be added. Never add the chemical to be reacted, in this case strontium nitrate to the strong acid or strong base. This could generate too much heat in that localized area causing the acid or base to boil and splash back at you. Always wear safety glass or goggles. I just picked up 8 of them from Amazon for $2 each for studio members using our new glass saw and glass grinder.

Stuff is real cheap on Amazon as they've recently relaxed the requirements for direct ship from China. Rather than buy a Bohl glass circle cutter for $85 I purchased a nearly identical Chinese version for $5.90, which included the shipping cost from Shenzhen?! Shenzhen is on the border with Hong Kong and is the official exit point for most goods produced through-out China, on their way to the rest of the world.

From memory I think Catherine Rehbein fires her SCM to cone 8, but you'd have to confirm this with her.

Comment by Robert Coyle

I'm not sure what people are looking for here. both Strontium acetate or chloride are bot very soluble in water, so they should be absorbed into the clay body and leave not much in the glaze.

Comment by Joseph Fireborn

I am new to this stuff haven't done chemistry in 15 years. I did add the vinegar to the strontium carbonate and it immediately foamed up and had a reaction like you stated. I did wear a safety mask. I will probably mix up the batch of scm with the acetate tonight and spray it in a few days.

Comment by Norm Stuart

Robert - In my experience Strontium Carbonate doesn't incorporate well into glazes at Cone 6.

There are special Strontium Frits available,  especially used for glazes at Cone 2 and below, but these are quite costly.

I'm actually headed to the studio right now to try vinegar and HCl on strontium carbonate. As for migrating into the bisque, that has benefits as well. Just add more strontium chloride.

Comment by Norm Stuart 

I would definitely would like to know what Cone is required to make Strontium Crystal Magic fire like Cathy Rehbein's work. I know from experience it's obviously much hotter than Cone 6 - the same with Jen's Juicy Fruit which is a non-event at Cone 6 oxidation.

Especially given the fact that this is at least nominally a Cone 6 website.

I found kitchen white vinegar easily bubbles off the CO2 from strontium carbonate, making it into strontium acetate which decomposes into Strontium Oxide at 459 F, much lower than Strontium Carbonate - although no one agrees on what that temperature is exactly.

I'm going to try this lower melting strontium in some glazes to see if they look different than the one which use strontium carbonate. The drawback is 43 grams of strontium acetate dissolve in a liter of water, so that will produce a different result as well. I different result I suspect may be quite useful based on the ceramic makers who put strontium frit into their clay bodies.

I've spent a lot of time reworking alleged Cone 6 glazes so they actually fire well at Cone 6. Pete Pinnells Weathered Bronze Green was one of them. Pete says it was originally a Cone 10 glaze and one of his students found it also "worked" at Cone 6. I found "worked" was too subjective and likely the result of an inaccurate kiln temperature which had to have been hotter than the student thought.

Making Weathered Bronze Green into a true Cone 6 glaze required the addition of 20% Ferro Frit 3269. After that addition it looks as beautiful as when it's fired to Cone 10. It only stands to reason it couldn't possibly look the same after firing to Cone 6 without a change in the recipe.

Comment by Teresa Wooden 

Hi all, been away for awhile.  Some health problems and a major move has taken me out of the game for awhile. Glad to be back.

I fire my work according to an old firing schedule of Steven's which he gave me several years ago.  Basically it fires to cone six, but with the holds and slow cooling the heatwork causes cone nine to bend.  Cone six is flat.  SCM and several other glazes I have work fine at that schedule, but recently I had to switch clay bodies and chose a cone six body... so I have tried to bring the actual firing temp down.  I am finding that SCM and a few other glazes do not melt at the lower temp.  My glazes don't look the same.  I am going back to the beginning and will be testing some alterations.  One thing I wonder is, since Custer Feldspar changed a couple years ago whether it may be altering the glaze.  It was one reason why I had to change clay bodies.   Another factor is that SCM needs to be applied very lightly...if applied thickly it looks unmelted and makes a mess.  Sometimes I draw a pencil mark down the side of a pot (especially porcelain..white on white) so that I can tell when I have sprayed just enough to cover the mark.   I will be sure to report back when/if we find any alternatives.  Peace to all.

[Norm Stuart]

Theresa - I'm glad to get some validation on these points regarding SCM and other allegedly Cone 6 glazes.  I wasted a lot of hours five years ago trying to make some of these recipes work at Cone 6.

I am finding that SCM and a few other glazes do not melt at the lower temp.  My glazes don't look the same.  I am going back to the beginning and will be testing some alterations.  One thing I wonder is, since Custer Feldspar changed a couple years ago whether it may be altering the glaze. - http://cone6pots.ning.com/xn/detail/2103784:Comment:135182?xg_sourc...

Custer Feldspar by itself doesn't melt at Cone 6, even when it had more potassium years ago. I doubt anyone could notice a change in the degree of melt based on a change in feldspar, only a change in the final COE or gloss.

This is what feldspars look like after being fired to Cone 11, about 130 degrees hotter than Cone 6. Only the high soda spar (Na - natrium) is showing any flow.

courtesy of Plainsman Clay via Digitalfire.

.

At Cone 6 feldspars are little more than clay, which do contribute salts equal to 10 to 14% of their weight, to the final melt.

But without adding Frit, or a low temperature flux like a boron item like gerstley borate, zinc, lead or bismuth the feldspar won't melt at all.

Even with 10% frit, feldspars are only just starting to act like a glaze at Cone 8. 

To make this mixture act like a Cone 6 glaze you'd need to increase the percentage of Frit to 20% to 26%.

.

[Joseph Fireborn]

Norm, 

This explains a lot of what I am experiencing. I have not had much success with SCM, almost always my surfaces have been super gritty and sandy feeling. So much so that I don't even enjoy touching the surfaces. I have tried thin layers of SCM as many have talked about, but then you don't get much out of it and the surface is very similar to the original glaze. 

Has anyone ever ran a currie grid test on SCM? I think I am going to do that soon. I have some grids drying that I will put under a fan tomorrow before I bisque them. I will run this SCM currie test and post results. I bet in the lower left corners of the grid I will find a better version with more melt, because of a much higher flux content. I imagine a lot of the tiles will be very dry. 

[Joseph Fireborn]

Also, does anyone have the correct schedule for cone 6 SH firing schedule? I have read so many different ones online. Even the one in his video's from CAD are different from the ones in Britt's book. 

[Tom Anderson]

Hi Joseph:

Have read through the posts regarding this topic. Looks like you are trying to alter the molecular structure of strontium through antistropic etching. I think in this particular case, KOH (potassium hydroxide) might be the better etchant, given the base molecule of silicon. Just happen to have a pound of silicon carbide and a few pounds of KOH out in the studio. I use this same technique on silica with good results. Give me a week or two to get results, and I will share them later. 

Tom

[Norm Stuart]

The underlying question is at what Cone does strontium participate in the glaze, as carbonate or other form. This is prompted by the observation that Sctrontium Crystal Magic doesn't appear to do much to a glaze at an actual Cone 6. Many using SCM fire one or two cones hotter.

Tom Anderson said:

Hi Joseph:

Have read through the posts regarding this topic. Looks like you are trying to alter the molecular structure of strontium through antistropic etching. I think in this particular case, KOH (potassium hydroxide) might be the better etchant, given the base molecule of silicon. Just happen to have a pound of silicon carbide and a few pounds of KOH out in the studio. I use this same technique on silica with good results. Give me a week or two to get results, and I will share them later. 

Tom