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Chris Campbell

Really, Really Basic Question

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PeterH    87

Chris,

 

You might also try adding borax to your mix, hopefully the boron will reduce the temperature

needed to produce effects (if that is a problem for you).

 

 

I attach a couple of entries from an old clayart posting at http://www.potters.org/subject14497.htm/

 

Regards, Peter

 

Peter Atwood on tue 12 jan 99
Got yet another question for you. I recently tried for a soda effect on a small bisqued test pot

by brushing a mixture of soda ash and water onto the pot. I used white stoneware so that I could

really see the final effect and applied the mixture liberally. Then I fired to ^10R.

The result was this: Shiny dark orange flashing on some areas, pale shiny finish on others, and

crusty yellowish barnacle-like blisters on the rest. The inside seemed to have burned away

completely and left nothing but bare clay.

I'd like to achieve a controlled orange flashing effect. Is there something else I could try to get this to

happen? Even if the flashing is sporadic, is there a concoction that will at least be devoid of blisters?

Any suggestions would be much appreciated.

Sharon Pollock-De Luzio on fri 15 jan 99
Try dissolving the soda ash very well with hot water and spray it on the pots. I have used a regular water

bottle sprayer. With trial and error you can learn to control this quite well. WORD OF CAUTION: soda ash

is very caustic. Wear respirator, gloves, eye protection, long sleeves.
 

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terrim8    79

Soda ash is sodium carbonate.  Baking soda is sodium bicarbonate.  Heat the latter above 50 degree celcius causes thermal breakdown of the bicarbonate. One CO2 molecule is released as gas, leaving sodium carbonate.  This a clip from Wikipedia.  You can also reference Chemistry.about.com and search sodium bicarbonate's thermal decomposition.   Most texts suggest heating the baking soda above 350 degrees F for 1 hour. 

 

Also, the soda ash will reabsorb CO2.  So it is best to store it in a sealed, air tight container.  Many of the problems using old soda ash in glazes comes from the fact that the soda ash has recaptured water and CO2. When this happens its molecular weight increases and thus changes the glaze chemistry balance.

 

Mark, I do know what I am talking on this one.  I have a major in Microbiology and Chemisty.

 

Thermal decomposition

Above 50 °C, sodium bicarbonate gradually decomposes into sodium carbonate, water and carbon dioxide. The conversion is fast at 200 °C:[9]

2 NaHCO3 → Na2CO3 + H2O + CO2

Most bicarbonates undergo this dehydration reaction. Further heating converts the carbonate into the oxide (at over 850°C):[9]

Na2CO3 → Na2O + CO2

These conversions are relevant to the use of NaHCO3 as a fire-suppression agent ("BC powder") in some dry powder fire extinguishers.

 

 

Gail Nicols uses "light soda ash" in her recipe for soda firing. This refers to calcining the soda ash.  So I'm not going from bicarbonate to carbonate. What temp & how long & what is the end product- the oxide? 

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soda ash is sodium carbonate.

 

The light vs dense soda ash is outlined here: (Solvay is one of several major suppliers of soda ash) -
http://www.solvay.com/en/markets-and-products/featured-products/soda-solvay.html

 

Light soda ash is a granular form that is best described  as "fluffy". 

It has a low bulk density and small particle size

 

Dense soda ash is less fluffy and larger particle size. 

 

The difference is that the light soda ash particles are porous and the dense soda ash particles are not porous.

 

Light soda ash dissolves more rapidly in water and worked best for Nichols' technique of making 'chunks' of a mixture of whiting and soda ash and water that solidified.  

 

If you are spraying soda ash either will work just as well.  

Soda ash is anhydrous when shipped by the manufacturers, but exposed to humid air it will slowly absorb water to form a soda ash hydrate.  The effect of this is that 100 grams of powder of a hydrated soda ash will have less 'active' ingredient than 100 grams of anhydrous material.  Hydration also causes the free-flowing powder to form somewhat hard lumps.

 

 

LT

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terrim8    79

Thanks.

It would be easier if our ceramics supply store was open next week.

 

I just found this process from a paper discussing the Solvay process on an industrial scale and they start with the bicarbonate.

 

"2.1.2.6. Sodium bicarbonate calcination Sodium bicarbonate cake is heated (160 to 230°C) to achieve calcination into a solid phase «light soda ash» and a gaseous phase containing CO2, NH3 and H2O. This gas is cooled to condense water and the condensates formed are sent to distillation for NH3 recovery, either directly or via filter wash water. After cleaning, the gas (high CO2 concentration) is compressed and sent back to the carbonation columns (CO2 recovery cycle in Figure 2). Normally, energy needed for sodium bicarbonate calcination is provided by steam that condenses in a tubular heat exchanger which rotates through the sodium bicarbonate. The method consisting of heating externally by gas or fuel oil combustion in a rotating drum containing sodium bicarbonate is occasionally encountered. "

 

I want to try it but I'm afraid the house will smell like ammonia!( I wonder if it would clean the oven) (can't afford a new oven) (going to wait for the store to open or just use the spray method)

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Terri and I are doing a soda firing next week, and the light version of soda ash isn't usually available without a special order. We had hoped to try Nichols' "chunk" method rather than spraying.

She mentions in her book that in workshop settings, she has calcined dense soda ash to about 600 F, and it works in a pinch to create the plaster she uses. She unfortunately does not give a time frame for this stuff to be in the kiln. The Wikipedia source states that sodium carbonate begins to decompose from the monohydrate, presumably turning anhydrous at around 212 F/100 C burning off whatever water it's absorbed from sitting on the shelf, and presumably becoming more porous than it was (oversimplified). My question would be is she suggesting the 600 F temperature to expedite the process because time is at a premium in a workshop setting? Can I calcine some dense soda ash in my oven at around 212F or do I have to mess around trying to figure out if my manual kiln with no pyrometer is at 600F because they don't make cone 023 or lower? (022 is roughly 1100 F, depending on firing rate.)

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Callie,

 

None of the readily available soda ash or baking soda have ammonia (NH3) in it. 

The solvay article is describing the 'solvay process' of making soda ash from salt water and sulfuric acid.  None of the soda ash one the market in the US or Canada comes from that route, it is too expensive to compete with the Trona mined in Wyoming.  So you don't have to worry about NH3.


I suggest that you take whatever grade of "soda ash" you have on hand, dry it in your oven at say 250 - 300 F for a couple of hours, or put it the kiln on low.  You do not want to melt it.  You also need to keep the oven/kiln ventilated to avoid  CO2 buildup.  Cool it back to room temperature and the place it in a blender and blend it to a fine powder.  Use the fine powder to make the Nichols' chunks.  I think the key to the chunks is to get the dry ingredients well mixed and not have too much water.  The chunks should keep without problems. 

  

Sidebar -- Measuring low temps <400F in an electric kiln without Thermocouple.  
  
   I monitor my manual test kiln with a grill thermometer that goes only to 450 F.  It sticks in a peep hole and gives me data on the early heating period when I fire the kiln.  For the soda ash drying, you can also place an oven shelf thermometer in the on the kiln shelf with the containers of soda ash you put in the kiln to calcine it.  Just keep a close look on what is happening.
 

Or you could skip the drying step and just try making the chunks with the materials you have.

 

If it works, then use the chunks in the firing; if not, just spray a soda ash solution.  Use draw rings, or an equivalent, to tell how well the ware has been glazed, etc., etc...

 

I have not tried the Nichols  chunks, but have studied the soda firing process extensively.  My contention is that the chunks slow the rate of soda vaporization from nearly instantly using the spray technique to a small amount of vapor spread  evenly (nearly) over the time from throwing the chunk into the kiln until the kiln is shut down.  The large body of evidence is that both can produce good work.

I remember an old Alka-Selzer ad that said "Try it, you'll like it." 

 

Good luck and have a safe and happy New Year.

 

LT

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Marcia Selsor    1,301

Speaking of the soda ash from Wyoming, I taught a soda kiln building workshop at Western Wyoming college back in the 80s and we used the local soda ash straight from the mine. Some over exuberant students poured a lot of soda ash through the top port in the catenary arch. When we opened the kiln, that ash had eaten a hole through the silicon carbide shelf. The pots came out very nicely!

Marcia

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glazenerd    816

250F is more than enough to drive off surface moisture (clumping). Soda ash is hygroscopic ( fancy word for "draws moisture from the air).  Calcining is done to drive off molecular H20: not needed in this case.

 

Absolutely do not mess with the Solvay process; of producing soda ash from sodium chloride and calcium carbonate Thinking this is where someone mistakenly got the 600F from??.  You do not need to be releasing chlorine gas in your work area. Sodium "chloride" is also the basis of hydro --"chloric" acid. Which is the reason Marcia shelf got eaten alive.(I suspect)

 

Sodium carbonate has been the basis of "soda glass" (window glass) for over a century. Safe, no toxic fumes.

 

Why not make a slip from soda ash and 2-3% bentonite: make droplets on a cookie sheet through your hydrometer: and bake them to dry. Should work.

 

Nerd

 

Edit:  sodium bicarbonate  Na2CO3  (CO3) off gasses.

Sodium chloride: NaCi- In the first oxidation state, is conversion to chlorine gas. The reason you do not want to be using the Solvay process or chloride/alkali (calcium) in your oven or kiln.

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terrim8    79

So....

I have 2kg of  granular soda ash. I can put it in a big bisque bowl, turn the oven on to 250 and check hour by hour for fluffiness? (that's my kind of baking) Then weigh it quickly & put it in an airtight container and save it to mix with the baking soda and limestone, right? 

 

GaIl NIcHolS Soda MIx

Light Soda Ash . . . . . . . . . . . . . . . . . . 20 Sodium Bicarbonate (baking soda) . . . 30 Calcium Carbonate . . . . . . . . . . . . . . 50

100 %

Add 9 U.S. fl. oz. of water per 1 lb. (600 ml of water per 1 kg) of dry mix. Wearing gloves, mix the dry ingredients thoroughly, then add the water all at once. Stir until the mixture begins to set, then break it into small pieces.

Light soda ash is required for the setting process. A wet mixture of sodium bicarbonate and calcium carbonate will not set; neither will a mixture using dense soda ash. Sodium bicarbonate is not an essential part of the soda source, but it makes the wet mixture less caustic. Sodium bicarbonate is also inexpensive and readily available in the supermarket or from a bakers’ supplier.

As this mixture breaks down in the heat of the flame, water vapor is released along with the vaporizing soda. Water vapor helps to carry the soda through the kiln chamber, enabling good glaze distribution and evidence of flame movement on the work. Water vapor also appears to assist with soda dissociation and glaze formation. "

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glazenerd    816

soda ash is Na2CO3     Baking soda is Na2HCO3    only difference is a (h) hydrogen binder; which burns off

Limestone is calcium carbonate (whiting).   some mineral differences as in SI/AL ratio

 

 

Sodium bicarbonate is an amphoteric compound. Aqueous solutions are very mildly alkaline due to the formation of carbonic acid and hydroxide ion:

carbonic acid is very mild, but an acid none the less- so gloves is a good idea. Better reason, it will suck the moisture out of your skin,

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Mark C.    1,807

I like spraying salt in liquid form. Our tries at spraying soda always plugged the sprayers so we made soda tacos-that is melted soda ash in hot water until thick cream and cream like and poured it into newsprint and folded into a taco shape and tossed these remade tacos into the worst inside bag walls while spraying slat into spray ports.

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The reason most potters have problems spraying soda ash (or salt) solutions into a kiln is that they are greedy about water, they don't use enough.  to make things easy, increase the water so that all the soda ash or salt is dissolved, and there are no crystals left in the mixing bucket.  Spray this solution into the kiln using a pressure sprayer so that the mist is fine, not large drops. 

You will find that you can get a very good glaze layer, have more control over where the light and heavy coatings occur, and have less damage to the kiln.  Also the additional water will increase the reaction between the soda/salt and the surface of the pots. 

The  over simplified reaction sequences you read in the pottery textbooks for salt/soda firing are not wrong, they just are weak on being right.  The chemistry has been well researched and thoroughly understood since the late 1980's by researchers studying the combustion of coal and biomass and the corrosion in gas turbines.  None of this work has been published in the literature read by potters.

LT

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Mark C.    1,807

The reason most potters have problems spraying soda ash (or salt) solutions into a kiln is that they are greedy about water, they don't use enough.  to make things easy, increase the water so that all the soda ash or salt is dissolved, and there are no crystals left in the mixing bucket.  Spray this solution into the kiln using a pressure sprayer so that the mist is fine, not large drops. 

You will find that you can get a very good glaze layer, have more control over where the light and heavy coatings occur, and have less damage to the kiln.  Also the additional water will increase the reaction between the soda/salt and the surface of the pots. 

The  over simplified reaction sequences you read in the pottery textbooks for salt/soda firing are not wrong, they just are weak on being right.  The chemistry has been well researched and thoroughly understood since the late 1980's by researchers studying the combustion of coal and biomass and the corrosion in gas turbines.  None of this work has been published in the literature read by potters.

LT

I'll try that next time . Our system has been to dissolve as much salt until there are no crystals then add a coup or two more of water and spray-this is all done with HOT water on huge pot on a burner.Maybe more water which cools down the kiln more and will take more recovery time can help us. Now we spray until temps drop way off and let it regain and repeat .We now have had zero issues with clogging with this technique. But as I said we make the soda into tacos as its 95% a salt fire only adding soda for extra foot colors

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JBaymore    1,432

 Can you recommend a good basic book that will not cause a brain freeze?

 

Chris,  Wow......... someone "liked" a post that I made in this thread ...and that led me to see that I never answered your question about a book.  I am SO sorry.

 

Believe it or not...... I happen to like an "old standby" as an 'intro to glaze chemistry' text.  It is not the latest and greatest... and some stuff is slightly dated... but it introduces the subject kind of nicely and gently.  Well written.  Hits both the materials sourcing side and the chemistry side.

 

Lots to build upon from there.... but it is a good start.

 

Sorry for the delay again.  My brain must have finally caught back up with my body from Japan.

 

best,

 

.................john

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oldlady    1,323

maybe not, john, what is the title of the book?

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JBaymore    1,432

maybe not, john, what is the title of the book?

 

I don't write these initials often........ OMG!

 

"Clay and Glazes for the Potter" by Daniel Rhodes.  Last version  updated by Robin Hopper.

 

best,

 

............john

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Marcia Selsor    1,301

Geez, John. Take a rest, get some sun and get over the Jet Lag. It is unlike you to be even a little off. You're worrying me.

Happy New year.

 

 

Marcia

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Most of my undergrad work was done in a soda kiln, but I graduated in 2001 and haven't been able to touch it since. It was before Gail Nichols research was published, and her plaster chunk method, hence my questions about the different qualities of the soda ash used.

 

The kiln I learned on was 50 feet from the school parking garage, so it was a dedicated soda kiln, no salt with the chlorine off-gassing allowed. Soda ash and bicarb are much more sluggish in the kiln by themselves. The sprayer usually had the metal tip melted shut from getting too close to the spy holes so I've never really used that method, although I understand it's pretty standard now. My favourite soda delivery method was to weigh out the soda I wanted to use (about 2 lbs/1kg in a well seasoned kiln) and dissolve it in hot water. I'd then add enough sawdust to absorb it all, and wrap it up into newspaper burritos and chuck those into the firebox at 15-20 minute intervals. The sawdust helped disperse things. It made for good flashing, and I liked the record of the flame path.

 

Marcia, I'm not gonna lie, I laughed out loud at your story! That sounded expensive. I hope no one was too angered about ruined work.

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Marcia Selsor    1,301

Now that I see you were shoveling snow...I understand. Sit back with a cup of tea and warm up. Tomorrow our high is -2 F.

We're loving it.

Marcia

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Marcia Selsor    1,301

Most of my undergrad work was done in a soda kiln, but I graduated in 2001 and haven't been able to touch it since. It was before Gail Nichols research was published, and her plaster chunk method, hence my questions about the different qualities of the soda ash used.

 

The kiln I learned on was 50 feet from the school parking garage, so it was a dedicated soda kiln, no salt with the chlorine off-gassing allowed. Soda ash and bicarb are much more sluggish in the kiln by themselves. The sprayer usually had the metal tip melted shut from getting too close to the spy holes so I've never really used that method, although I understand it's pretty standard now. My favourite soda delivery method was to weigh out the soda I wanted to use (about 2 lbs/1kg in a well seasoned kiln) and dissolve it in hot water. I'd then add enough sawdust to absorb it all, and wrap it up into newspaper burritos and chuck those into the firebox at 15-20 minute intervals. The sawdust helped disperse things. It made for good flashing, and I liked the record of the flame path.

 

Marcia, I'm not gonna lie, I laughed out loud at your story! That sounded expensive. I hope no one was too angered about ruined work.

No. It was a good learning experience. The one silicon carbide shelf didn't have that big of a hole... maybe 3/4 of an inch. Just a direct hit with soda straight from the soda ash mine. Got some nice pieces out of it. That was in 1987.The impure soda ash gave interesting results.

 

Marcia

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terrim8    79

Too bad about the Solvay process as this oven really needs cleaning & I'm lazy! Bisque bowl wasn't big enough for 2kg so cheap casserole it is. Now how many people in North America are slow roasting soda ash at 250F in their ovens right now? (besides Callie)

post-67248-0-24630500-1483303727_thumb.jpg

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Lots of people are calcining soda ash in their ovens, especially those baking biscuits, cakes, and cookies :D .


Now:
A little bit of simplified chemistry and physics to lull you asleep while the soda ash is roasting.

Look at the soda ash:

For soda ash to vaporize, it must react with water to produce NaOH and CO2. The reaction is Soda Ash (solid/melted) + Water => sodium hydroxide (vapor) + Water (vapor).  These two gaseous species become part of the combustion gases and react rapidly with the bare surface of the pots.  That reaction is 2NaOH (vapor) + clay body = glaze (Na2O-claybody) + water vapor.

Look at a spot on the pot:

When the spot is bare hot clay body, the NaOH reacts as described above with the bare spot to cover the spot with glaze.  The surface of the spot is now not bare and the surface that the NaOH (vapor) sees is no longer clay body it sees glaze.  The reaction becomes 2NaOH (vapor) + Na2O-dissolved-in-glaze (Na2O-claybody) + water vapor.  The dissolved Na2O must migrate through the glaze to reach the clay body underneath.  Glaze grows inward.   This migration takes time, just like it takes time to migrate through a crowd to get to the drinks table at a party. The thicker the crowd (in this case the glaze layer) the longer you wait for a drink.  This is why after the first dose of soda, it takes longer to buildup the glaze to thickness. The thicker the glaze gets, the longer it takes to grow thicker.

Look at spraying a soda ash solution as a fine mist into the hot gases. 

If the droplets are tiny (less than 10 micron say) they will be quickly heated and the water/soda ash will react as described above to form a small pocket of vapor of NaOH that will be carried along with the combustion gases.  This will happen in a time frame in tens of milliseconds not in seconds.  If the drops are large (1 mm say) the size and momentum of the droplet will be enough for the droplet be slowed and fall to the floor or hit a wall or a pot before evaporating.  On the floor, wall, shelf, or pot the reaction described above will also take place. Hitting anything other than a pot essentially defeats the purpose of putting the droplet into the kiln.

 

Looking at the Nichols' chunk delivery of soda ash:

The Nichols' chunks (plaster as Callie called it) is in the fire box on the floor.  The flames in the fire box heat the chunks to the temperature of the refractory floor of the fire box. The baking soda is calcined to soda ash, the soda ash melts and coats the whiting particles and waits for water vapor to arrive to vaporize per the reaction(s) described above.   The whiting is just "filler" that helps decompose the sodium carbonates and keeping the sodium carbonate melt off the floor.  Water vapor from the combustion gases must migrate into the chunks, react with the soda ash/baking soda, and the NaOH(vapor) must migrate out into the main stream of the hot gases leaving the fire box.  The overall effect essentially puts a steady slowly vaporizing source of NaOH vapor leaving the fire box into the ware chamber.

 

Looking at 'soda ash tacos' delivery:

The tacos fall to the floor and must be heated to the floor temperature.  The saw dust burns out and may shorten the time for the taco to reach temperature.  Much of the water will vaporize without reacting with the soda ash. The soda ash will melt and spread as a thin corrosive liquid over the floor and into the cracks in the refractory, otherwise the reactions described above take place.   
 

comparison of Nichols chunks and tacos and spraying :
 
Nichols chunks are probably a little less corrosive to the kiln than the 'tacos' of soda ash and sawdust as the whiting acts as a barrier between the molten soda ash and the floor. 
 
The chunks certainly don't cause the rapid temperature drop from the water in the spraying method.  The spray method provides a water rich region to facilitate the vaporization reaction of soda ash + water.  It does cause a drop in local temperature for a minute or two.   The spray method gives the potter a means of being selective where soda is directed (assuming that ports are available). 

All of the methods require heat from the combustion process to bring the reagents injected to the kiln to kiln temperature and to vaporize the soda ash to NaOH vapor.  This effect can be predicted or estimated from previous firings and properly planned for.

 

The spray method allows the potter to focus on some regions more than other regions of the kiln.  It also require more effort during the firing with all the spraying and movement around the kiln.  It needs ports to access the area where the spray does the most good.
 
One method does not preclude using the other. 
 
A key point alluded to by all the above is that the combustion gas is the  carrier that moves the NaOH vapor around and through the kiln to the pots.  Without the combustion gas and its movement, the soda ash would just sit in a puddle on the floor.  Therefore sealing off the kiln to keep the "soda" in the kiln longer is likely to be counter-productive, but changing the flow patterns may be useful in obtaining interesting effects on the ware.

LT

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terrim8    79

Do the chunks get delivered into the kiln the same way as the tacos?  i.e. sliding down a rod of angle iron into the kiln thru a side port. And altering the flow pattern is done just by adjusting the flue once in awhile? Callie has done this but I have only participated in a crowded group event and like your bar scenario for glaze migration, I was often at the back of the crowd.

We intend to fire all night to get to cone 9/10, likely starting reduction at 012. That way we will do the soda /draw rings repeats in the bright, cold (except near the kiln) sunny  mid-morning the next day. Working around a kiln in the winter always makes me think of Sam McGee.

BTW-  The soda ash has been at 250F all afternoon &  is still granular so I just put the temp up to 300. We are at 3428 ft ASL here so maybe it just takes longer.

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