glazenerd

Tim:
Temper is well known among those who collect and process natural clay; and those who fire primitive pottery. Temper is added for malleability; in lieu of known clay formulation methods. Grog is added to increase green or fired strength, or it can be added to control warping, and or increase thermal shock properties.  Hardpanning is more associated with glaze than clay; although it can happen in soil deposits if conditions are right. Processing natural clay rarely comes up on this forum: although it has gained popularity in the last decade. 
Tom

I've tried making paper clay in very small amount to try using for patching but was not impressed...I did have a customer who wanted me to incorporate his parent's ashes into the surface of an urn he wanted me to make for him. This was the result:

It has been close to a decade since I bought any commercial clay. Years ago, I put the same crystalline glaze on five different commercial porcelain bodies, and got five different results. That single incident sparked my curiosity about clay formulation; which turned into an obsession for awhile. I do crystalline tile almost exclusively; and every commercial porcelain body I tried- warped. From that, I developed a crystalline body that actually promotes crystalline growth; instead of hindering it. I developed a porcelain tile body that I roll out on 2 x4 x 1/2" luan, and let it sit until it pops off on its own. My days of flipping, weighting, and spending hours trying to prevent warping are long over. Few years back I developed an extensive line of colored porcelain: which I use from time to time. Local supplier wanted me to distribute it; but not really had the time to get that involved. A shoulder injury from a few years back has also hindered that endeavor.
My bigger interest of late has been collecting and processing locally sourced clay. It holds my interest; breaking it down and figuring out how to make it a workable body. I have samples sent to me from across the US, and on rare occasion- other countries. Rather fascinating to see the results from iron disulfide, hematite, and magnetite samples. Still toying with the idea of bringing back a semi-load of hematite clay from NE Oklahoma- best looking cone 6 results I have ever seen. I have been contacted by potters across the globe for in depth discussions on how to fix their naturally sourced clay issues. The most complicated puzzle was sub-tropical laterite from India; took awhile to find the solution. (43% total iron/alumina content). Collecting and processing natural clay is more popular than most realize; but it is not a topic that is widely discussed here.
A few months back another interest has taken center stage in my life: so clay may have to take a back seat for awhile.
Nerd -- the pic below is Oklahoma hematite.

It has been close to a decade since I bought any commercial clay. Years ago, I put the same crystalline glaze on five different commercial porcelain bodies, and got five different results. That single incident sparked my curiosity about clay formulation; which turned into an obsession for awhile. I do crystalline tile almost exclusively; and every commercial porcelain body I tried- warped. From that, I developed a crystalline body that actually promotes crystalline growth; instead of hindering it. I developed a porcelain tile body that I roll out on 2 x4 x 1/2" luan, and let it sit until it pops off on its own. My days of flipping, weighting, and spending hours trying to prevent warping are long over. Few years back I developed an extensive line of colored porcelain: which I use from time to time. Local supplier wanted me to distribute it; but not really had the time to get that involved. A shoulder injury from a few years back has also hindered that endeavor.
My bigger interest of late has been collecting and processing locally sourced clay. It holds my interest; breaking it down and figuring out how to make it a workable body. I have samples sent to me from across the US, and on rare occasion- other countries. Rather fascinating to see the results from iron disulfide, hematite, and magnetite samples. Still toying with the idea of bringing back a semi-load of hematite clay from NE Oklahoma- best looking cone 6 results I have ever seen. I have been contacted by potters across the globe for in depth discussions on how to fix their naturally sourced clay issues. The most complicated puzzle was sub-tropical laterite from India; took awhile to find the solution. (43% total iron/alumina content). Collecting and processing natural clay is more popular than most realize; but it is not a topic that is widely discussed here.
A few months back another interest has taken center stage in my life: so clay may have to take a back seat for awhile.
Nerd -- the pic below is Oklahoma hematite.

Maybe of interest ...
Temper (pottery) https://en.wikipedia.org/wiki/Temper_(pottery)
A temper is a non-plastic material added to clay to prevent shrinkage and cracking during drying and firing of vessels made from the clay.
... and goes on to list 12 types
I don't know how useful this definition is, or how widely it would be accepted.
Note that the definition doesn't cover the use of  "combustibles" such as coffee grounds or expanded perlite to add texture to the fired surface.
Are there any additions to a flameware body purely to modify the fired thermal properties, rather than the shrinkage/cracking of the body during drying and firing? [Mica might be one?]
 
PS It doesn't pass modern  health & safety requirements -- and it has nothing to do with your questions -- but have you heard of  Obvara Raku? Which I believe has it's origins in sealing porous cooking vessels.

Marcia Selsor's Tips on Obvara Firing
https://ceramicartsnetwork.org/daily/article/Marcia-Selsors-Tips-on-Obvara-Firing
The obvara firing technique is a technique originally used to seal low fire pottery. It is believed to have originated in Eastern Europe during the Middle Ages.
 


 
 
 
 

Hi Tina:
Been following, going through the information you posted. Sand additions are called "temper" in our clay world: first used in native pottery, and later by the brick industry to make clay more suitable for pressing. While sand added to the problem; it is not the problem. You soaked your clay for several days and still had lumps: this indicates the real problem- it is called "cementing." Cementing is a severe form of flocculation; caused by high iron/alumina levels that create a strong positive charge in the clay which makes it resistant to uniform dispersion of particles: which in turn results in delamination, sheering, and cracking. It requires more than adding plasticizers to the clay: you have to break the strong positive charge that creates this problem. Acidity creates flocculation and alkalinity creates deflocculation.
Now lets put that theorem to the test: 1. Find two 1" hard clumps of clay. 2. Fill 2 glasses, bowls, dishes with a 1/2 cup of tap water. 3. Add 1/8 teaspoon of sodum or potassium feldspar to just one  glass (leave the other plain tap water). If you do not have feldpsar; then add 1/8 teaspoon of baking soda.  Stir it well to disperse the powder. 4. Drop one chunk of clay into each glass/bowl. (Do not stir or agitate.) Let it stand for 30 minutes. 5. Use your finger or spoon to see if the clay chunk has dissolved on its own.
If my theorem is correct: then the chunk of clay in the glass with the feldspar/baking soda will have dissolved on its own.  Feldspars/ baking soda create alkalinity; which in turn creates a negative charge in the water; which in turn neutralizes the positive charge that created the "cementing" property.
Tom

"Sticky" is an indication that organics are present. A particular kind of humus called Ord is commonly found in natural clay. Ord is many times more plastic than bentonite: which makes clay "sticky." Have you dissolved any in water? If so, any dark particles float to the top? Without pics, I have to assume your "dark" description means dark gray? If so, most likely you have magnetite (iron) in your clay. Magnetite will fire a terra cotta color at cone 06, and turn deep brown at cone 6. Iron disulfide (pyrite) also follows that color transformation. Hematite (iron) will keep more of its terra cotta color at cone 6. Sheering in this case is most likely a lack of fine particle clay: which you indicated it has a sandy feeling. Add 20% OM4 or equal ball clay, and see if the sheering improves. Remember to let is sit 5-7 days after you mix it in; takes a bit for plasticity to develop.
Yes, a sheen can be an indication of vitrification: but it can also occur on the surface from out gassing of inorganic materials. I am making the assumption that is magnetite from your color description. The most common iron source in the USA is iron disulfide (pyrite); which on occasion can be a dark grwy color pending organic levels or other minerals in the deposit. If you fire iron disulfide quickly, you will get a sheen from sulfur off gassing. Without pics, sort of a guessing game at this point.
Tom

"Sticky" is an indication that organics are present. A particular kind of humus called Ord is commonly found in natural clay. Ord is many times more plastic than bentonite: which makes clay "sticky." Have you dissolved any in water? If so, any dark particles float to the top? Without pics, I have to assume your "dark" description means dark gray? If so, most likely you have magnetite (iron) in your clay. Magnetite will fire a terra cotta color at cone 06, and turn deep brown at cone 6. Iron disulfide (pyrite) also follows that color transformation. Hematite (iron) will keep more of its terra cotta color at cone 6. Sheering in this case is most likely a lack of fine particle clay: which you indicated it has a sandy feeling. Add 20% OM4 or equal ball clay, and see if the sheering improves. Remember to let is sit 5-7 days after you mix it in; takes a bit for plasticity to develop.
Yes, a sheen can be an indication of vitrification: but it can also occur on the surface from out gassing of inorganic materials. I am making the assumption that is magnetite from your color description. The most common iron source in the USA is iron disulfide (pyrite); which on occasion can be a dark grwy color pending organic levels or other minerals in the deposit. If you fire iron disulfide quickly, you will get a sheen from sulfur off gassing. Without pics, sort of a guessing game at this point.
Tom

"Sticky" is an indication that organics are present. A particular kind of humus called Ord is commonly found in natural clay. Ord is many times more plastic than bentonite: which makes clay "sticky." Have you dissolved any in water? If so, any dark particles float to the top? Without pics, I have to assume your "dark" description means dark gray? If so, most likely you have magnetite (iron) in your clay. Magnetite will fire a terra cotta color at cone 06, and turn deep brown at cone 6. Iron disulfide (pyrite) also follows that color transformation. Hematite (iron) will keep more of its terra cotta color at cone 6. Sheering in this case is most likely a lack of fine particle clay: which you indicated it has a sandy feeling. Add 20% OM4 or equal ball clay, and see if the sheering improves. Remember to let is sit 5-7 days after you mix it in; takes a bit for plasticity to develop.
Yes, a sheen can be an indication of vitrification: but it can also occur on the surface from out gassing of inorganic materials. I am making the assumption that is magnetite from your color description. The most common iron source in the USA is iron disulfide (pyrite); which on occasion can be a dark grwy color pending organic levels or other minerals in the deposit. If you fire iron disulfide quickly, you will get a sheen from sulfur off gassing. Without pics, sort of a guessing game at this point.
Tom

It has been close to a decade since I have purchased pre-mixed clay. I mix my own blends, using recipes I developed. I use a PP SS30 to mix. I have an electric cement mixer that holds 150 lbs of dry materials. I load it, let it mix for 15-20 minutes, and scoop out dry blend as needed. I mix outside to keep the dust down. I mix dry materials and water in a 3 gal bucket to create a heavy slurry: I do not pour water directly into the mixing chamber. I fill the chamber completely with dry mix, and then begin adding slurry. Once I get close, deair, then pug about 12” of clay, throw it back into the chamber to remix. This assures the chamber is filled with solid, well mixed clay. I mix mostly porcelain, an occasional stoneware; and done a few loads of colored porcelain. More recently, mixed several loads of locally harvested clay. I have marks on my table to lay pugs on: these marks allow me to cut 1-2-3-5 lb balls without weighing. 
Tom

It has been close to a decade since I have purchased pre-mixed clay. I mix my own blends, using recipes I developed. I use a PP SS30 to mix. I have an electric cement mixer that holds 150 lbs of dry materials. I load it, let it mix for 15-20 minutes, and scoop out dry blend as needed. I mix outside to keep the dust down. I mix dry materials and water in a 3 gal bucket to create a heavy slurry: I do not pour water directly into the mixing chamber. I fill the chamber completely with dry mix, and then begin adding slurry. Once I get close, deair, then pug about 12” of clay, throw it back into the chamber to remix. This assures the chamber is filled with solid, well mixed clay. I mix mostly porcelain, an occasional stoneware; and done a few loads of colored porcelain. More recently, mixed several loads of locally harvested clay. I have marks on my table to lay pugs on: these marks allow me to cut 1-2-3-5 lb balls without weighing. 
Tom

Yep an old modified plaster mixer outside and a pugmill. The clay ingredients from my formula are weighed out in proportion and put into the mixer in 100 or 200lb batches and dry mixed for a few minutes then water is added. Clay pulled out and put in 40 gal trashcans and wheeled inside and run thru the pugmill. Pugs of clay are placed in another trashcan and covered with plastic at each layer. Been doing it for years. I once bought a box of clay in the late 80s but none since.

It has been close to a decade since I have purchased pre-mixed clay. I mix my own blends, using recipes I developed. I use a PP SS30 to mix. I have an electric cement mixer that holds 150 lbs of dry materials. I load it, let it mix for 15-20 minutes, and scoop out dry blend as needed. I mix outside to keep the dust down. I mix dry materials and water in a 3 gal bucket to create a heavy slurry: I do not pour water directly into the mixing chamber. I fill the chamber completely with dry mix, and then begin adding slurry. Once I get close, deair, then pug about 12” of clay, throw it back into the chamber to remix. This assures the chamber is filled with solid, well mixed clay. I mix mostly porcelain, an occasional stoneware; and done a few loads of colored porcelain. More recently, mixed several loads of locally harvested clay. I have marks on my table to lay pugs on: these marks allow me to cut 1-2-3-5 lb balls without weighing. 
Tom

It has been close to a decade since I have purchased pre-mixed clay. I mix my own blends, using recipes I developed. I use a PP SS30 to mix. I have an electric cement mixer that holds 150 lbs of dry materials. I load it, let it mix for 15-20 minutes, and scoop out dry blend as needed. I mix outside to keep the dust down. I mix dry materials and water in a 3 gal bucket to create a heavy slurry: I do not pour water directly into the mixing chamber. I fill the chamber completely with dry mix, and then begin adding slurry. Once I get close, deair, then pug about 12” of clay, throw it back into the chamber to remix. This assures the chamber is filled with solid, well mixed clay. I mix mostly porcelain, an occasional stoneware; and done a few loads of colored porcelain. More recently, mixed several loads of locally harvested clay. I have marks on my table to lay pugs on: these marks allow me to cut 1-2-3-5 lb balls without weighing. 
Tom

Dexter:
perhaps I should explain the chemistry behind charcoal. The briquettes become granular when you crush them- 1/16, 1/8, 1/4": which acts as the grog to give this simple mix the strength to form. This is the reason grog, straw, and hair is not necessary. Rocket stoves are typically 1 1/2" thick: self supporting when formed. Oven ware bodies are primarily ball clay; that utilize low expansion fluxes (lithia/magnesium), not potassium/sodium used in cone 3 and up. Magnesium has a low melting point as well. Secondly, unmelted silica is highly expansive: so oven ware (and raku) swap out pyrophyllite for silica additions to help control expansion. However, kaolin is a poor choice for this type of project. The green strength of kaolin (porcelain) hovers around 400 psi, while ball clays approach 800 psi. 
T

Hi Dexter;
Your tandoor is a version of a rocket stove (google).  You are dealing with expansion (heating) and contraction (cooling.) In this case: low thermal expansion.  The silica sand is typically 30-40 mesh (Home Depot) is to help control expansion by creating voids in the clay body; for the same reason sheeps hair is used. The hair will burn out, leaving voids. Voids will absorb expansion that would normally cause issues in traditional pottery. There is a simplier way to get there: find the lowest expansion commercial clay available; these are normally sold as "ovenware" bodies.  Grab a bag of charcoal; yep, the BBQ stuff. Grind up the briquettes into a powder (wear a mask, outside). 30-40% is typical. For easy math- a 25lb bag of charcoal will do 75lbs of wet clay. When fired, the charcoal burns out, creating voids. Same principle, same result- much easier.  
Tom

Hi Dexter;
Your tandoor is a version of a rocket stove (google).  You are dealing with expansion (heating) and contraction (cooling.) In this case: low thermal expansion.  The silica sand is typically 30-40 mesh (Home Depot) is to help control expansion by creating voids in the clay body; for the same reason sheeps hair is used. The hair will burn out, leaving voids. Voids will absorb expansion that would normally cause issues in traditional pottery. There is a simplier way to get there: find the lowest expansion commercial clay available; these are normally sold as "ovenware" bodies.  Grab a bag of charcoal; yep, the BBQ stuff. Grind up the briquettes into a powder (wear a mask, outside). 30-40% is typical. For easy math- a 25lb bag of charcoal will do 75lbs of wet clay. When fired, the charcoal burns out, creating voids. Same principle, same result- much easier.  
Tom

Hi Dexter;
Your tandoor is a version of a rocket stove (google).  You are dealing with expansion (heating) and contraction (cooling.) In this case: low thermal expansion.  The silica sand is typically 30-40 mesh (Home Depot) is to help control expansion by creating voids in the clay body; for the same reason sheeps hair is used. The hair will burn out, leaving voids. Voids will absorb expansion that would normally cause issues in traditional pottery. There is a simplier way to get there: find the lowest expansion commercial clay available; these are normally sold as "ovenware" bodies.  Grab a bag of charcoal; yep, the BBQ stuff. Grind up the briquettes into a powder (wear a mask, outside). 30-40% is typical. For easy math- a 25lb bag of charcoal will do 75lbs of wet clay. When fired, the charcoal burns out, creating voids. Same principle, same result- much easier.  
Tom

Hi Dexter;
Your tandoor is a version of a rocket stove (google).  You are dealing with expansion (heating) and contraction (cooling.) In this case: low thermal expansion.  The silica sand is typically 30-40 mesh (Home Depot) is to help control expansion by creating voids in the clay body; for the same reason sheeps hair is used. The hair will burn out, leaving voids. Voids will absorb expansion that would normally cause issues in traditional pottery. There is a simplier way to get there: find the lowest expansion commercial clay available; these are normally sold as "ovenware" bodies.  Grab a bag of charcoal; yep, the BBQ stuff. Grind up the briquettes into a powder (wear a mask, outside). 30-40% is typical. For easy math- a 25lb bag of charcoal will do 75lbs of wet clay. When fired, the charcoal burns out, creating voids. Same principle, same result- much easier.  
Tom

Have 4 Paragons, and hit this issue on 3. The wires from the thermocouple to the block are constricted in a narrow channel, typically less than 1/16th separates them. The heat caused enough expansion that caused the wires to touch. Ended up putting a small piece of fiber insulation between them, after I separated them (very carefully). 
Tom

Granular Magnetite (iron) at 2%.  From wet clay calculate 20% as water. 10lbs minus 20% water(2lbs) = 8lbs. Dry x 2% granular Magnetite. Judging from pic- 20 mesh speckles.  Make a 500 gram test batch and fire as a tile with usual glaze to judge speckle population. Add or subtract as per personal preference. Iron will be safer in your case given kiln location: granular manganese can fume. 

Granular Magnetite (iron) at 2%.  From wet clay calculate 20% as water. 10lbs minus 20% water(2lbs) = 8lbs. Dry x 2% granular Magnetite. Judging from pic- 20 mesh speckles.  Make a 500 gram test batch and fire as a tile with usual glaze to judge speckle population. Add or subtract as per personal preference. Iron will be safer in your case given kiln location: granular manganese can fume. 

Never had an instructor: then again I never intended to go down the rabbit hole this far either. I noticed early on in another forum where I lurked mostly: there would be ten different answers to one question. I understood that application and style could vary from potter to potter: but never made sense on the chemistry end of the equation. So I made the decision early on to seek out information by those with PhD behind their names. Oddly enough, found variance there as well- who knew? If I had a mentor; that would be Ron Roy: who I had the pleasure to spend three days with at NCECA in KC (2016). We email back and forth to this day. I have been asked several times to do classes in St. Louis- to date I have always declined. My wife, siblings, and two friends even know that I am involved with clay. Has always been my private sanctuary: and never discuss it locally.
Tom

Oddly enough: I would like to spend a day unseen in a corner, watching a newbie throw their first form, open their first kiln firing, or sell their first cup. The joy and excitement is contagious. Would have loved to spend a day with W.G. Lawrence (Phd@ Alfred) and pick his brain. Same for Orton Jr, Koppatchu, Horton, and a few others who laid the foundation on which modern pottery is built. 

Oddly enough: I would like to spend a day unseen in a corner, watching a newbie throw their first form, open their first kiln firing, or sell their first cup. The joy and excitement is contagious. Would have loved to spend a day with W.G. Lawrence (Phd@ Alfred) and pick his brain. Same for Orton Jr, Koppatchu, Horton, and a few others who laid the foundation on which modern pottery is built.