Red Body Clay Guide

[glazenerd]

 

Notes: all test samples shown are blended at 73% clay, 12% C&C ball clay, and 15% mahavir potash as a baseline:  fired in oxidation.

 

Red body is a generic term used to describe iron bearing clays that have a red hue. There are other iron bearing clays that can present as green, grey, and black; these are exceptions, not the rule for commercial bodies. The three iron sources found in natural clay are hematite, magnetite, and iron disulfide. Iron disulfide is the common iron source in the USA and Canada; however hematite and magnetite are included because they are sourced in other countries. Hematite and magnetite are also harvested by local potters in North America. Typical iron bearing clays average between 5.0 to 8.4% of iron by weight.

The tile graph below shows unglazed red bodied clays, with a locally sourced magnetite ( dark gray) sample from NY. (TY Mary)  All five fire to a traditional Tera Cotta at cone 04; although there is some variance in color depth. At cone 3, enough heat is present to cause some reduction resulting in color shifts. At cone 6; iron disulfide typically turns a deep brown; magnetite is nearly black; while hematite maintains a Terra Cotta color pending alumina levels, or a deep reddish brown. The note below identifies the labels for each clay; as well as their iron source.

 

Additional Test Note: IM (IMCO Burgundy), and RA (Red Art) contains iron disulfide as the primary iron source. Newman Red (N) is a blended material that incorporates primarily hematite. H (hematite) is a locally sourced material that is not commercially available. Magnetite (M) is also locally sourced and not available commercially.

 

Nerd

[glazenerd]

 

The starting point for red bodied clays is the color when moist. The most commonly used clays in North America are IMCO Burgundy, Newman Red, and Red Art,  in addition magnetite, and hematite are locally sourced and or used in other countries. Iron bearing clays can also be grey (Canada), or have a distinct green hue ( Southern USA).  Green hue typically indicates that higher levels of calcium and magnesium are present. Grey can be  higher magnesium; but most often the color comes from sulfide contaminants (lignite coal particles). Red Art also has a high sulfide content which darkens the red hue.  Clays with high sulfide content require different firing schedules than hematite or magnetite bearing clays. Notice IM and RA have reddish brown and reddish/ grey color when wet: both discoloration indicate clay impurities that include lignite coal particles.

 

Ledger: IM- IMCO Burgundy. N- Newman Red  H- hematite  RA- Red Art M-magnetite.

 

[glazenerd]

Any red body clay sources iron to obtain its primary wet color. Iron disulfide with additional sulfides from lignite coal particles can have dramatic effects on colored glazes. The picture below shows yellow, red, green, and turquoise stains mixed with a zinc free clear glaze. The Mason Stain color number is noted on the sides. The pure porcelain tile to the left illustrates the dramatic color shift iron causes. The magnetite and Red Art samples cause enough reaction to render stains unusable. Porcelain on far left was used as a color control reference.

Ledger: IM- IMCO Burgundy. N- Newman Red  H- hematite  RA- Red Art M-magnetite.

[glazenerd]

 

An additional test was performed to check the reaction of the three iron sources to other metalloids. Copper carbonate was added to a zinc free clear glaze, then a zinc free clear glaze: then finally 3% zinc was added to the clear base glaze. Copper carbonate had strong reactions to all iron varieties in comparison to the porcelain control tile. Zinc free clear glaze performed well, with the exception of the magnetite (M) sample.  When zinc was added at. 3%: there was a color shift on all samples; with crystallization occurring on the IM sample. Magnetite had the strongest reaction to zinc with a wide color shift and blistering occur.

Ledger: IM- IMCO Burgundy. N- Newman Red  H- hematite  RA- Red Art M-magnetite.

 

When using clear glazes on red bodied clay bodies; there are often complaints about matting. In this close up (3rd pic) of a zinc free (left) and zinc added (right) crystallization of the iron  has occurred; which is often mistaken for matting. Iron crystals refract light differently, giving the appearance of matting. Where the clay and glaze meet is commonly known as the clay- glaze interface: enough of the iron has been pulled out of the clay body into the glaze to cause iron crystals to form. In addition: red clay bodies that incorporate iron disulfide as the iron source can cause matting if improperly fired. 

 

 

Close up of zinc free and zinc added clear glazes and the reaction to iron varities.

[glazenerd]

If red body clays that source iron disulfide are improperly fired; the off gassing inorganic gases can create color shifts so dramatic it renders the glaze useless. Porcelain control tile on left: iron disulfide samples in middle and on the right.

 

[glazenerd]

Firing Red Bodies with iron disulfide.

 

Red bodies that source iron disulfide, and or red bodies that have inorganic sulfides require changes in the firing cycle to prevent blistering, bloating, and carbon coring. (Also called black coring) Hematite and magnetite have oxygen binders that creates the crystal lattice. Iron for example is FeO Fe= iron. O = oxygen. Firing clay bodies that source these two iron varieties, cause few issues. Most red clay bodies in the USA and other places source iron as iron disulfide. (FeS) Fe= iron S= sulfur/ide. When firing red body iron disulfide special attention to the firing schedule is critical. Disulfide is an inorganic binder that off gases between 1250F (655C) to 1750F (955C). For this singular reason all red bodies needs to be bisq fired to 1800F (1000C) minimum.

 

Edward Orton (the cone guy) did the early research work on iron bearing clays from 1904 to early 1920's. He wrote numerous ACER journals on the kiln firing techniques required to properly fire and burn off the sulfides in red clay bodies. The slow ramp cycle on Orton controllers is based on his research. In lieu of the slow bisq ramp: a programmed ramp from 1250F to 1750F at 108F an hour climb is required to successfully burn out the sulfide content. Pending the size of your piece (over 10 lbs. or over 3/8" walls) you may need to slow down to 80F an hour between 1250F to 1750F. If you single fire red bodied clays: simply choose slow glaze fire or program a firing segment from 1250F (655C) to 1750F (955C) at 108F an hour climb.

[glazenerd]

Color development can also be achieved by changing your firing speed. Most controllers have slow, medium, and fast preset firing speeds. By simply changing from fast to medium firing speeds; additional color development can be achieved. Red bodies containing iron disulfide should be fired as recommended to bisq temperatures 1800F/ 1000C. Glaze firing can then be adjusted to fast or medium for color development in the glaze fire.

[liambesaw]

*grabs his popcorn*

[Bill Kielb]

53 minutes ago, liambesaw said:

*grabs his popcorn*

Kitchen run! Back for more.

[liambesaw]

This is great @glazenerd, is it safe to assume during a long bisque for FeS clays, we should vent or unplug a bung or two for extra oxygen?  I'm thinking that in addition to any carbons swooping up oxygen during combustion, that also the sulfides in the clay also need oxygen to convert to sulfur dioxide and gas out?

I've had sulfide issues like you've shown, on a previously used (lizella) body, with the special slow bisque, when packed tight, stacked, with the peeps in.  I've now switch to 2 peeps out til 1800 and have had a lot better results.  But I also haven't gone back to that specific clay body.

Thanks for the write up

 

[Bill Kielb]

1 hour ago, liambesaw said:

This is great @glazenerd, is it safe to assume during a long bisque for FeS clays, we should vent or unplug a bung or two for extra oxygen?  I'm thinking that in addition to any carbons swooping up oxygen during combustion, that also the sulfides in the clay also need oxygen to convert to sulfur dioxide and gas out?

I've had sulfide issues like you've shown, on a previously used (lizella) body, with the special slow bisque, when packed tight, stacked, with the peeps in.  I've now switch to 2 peeps out til 1800 and have had a lot better results.  But I also haven't gone back to that specific clay body.

Thanks for the write up

 

Do you slow bisque to a cone? 04/05

[liambesaw]

7 minutes ago, Bill Kielb said:

Do you slow bisque to a cone? 04/05

Yep!  Slow bisque to 06 with a 1 hour hold.  It drops the 04 and gets rid of the gasses.

[glazenerd]

On 4/29/2020 at 12:17 AM, liambesaw said:

This is great @glazenerd, is it safe to assume during a long bisque for FeS clays, we should vent or unplug a bung or two for extra oxygen?  I'm thinking that in addition to any carbons swooping up oxygen during combustion, that also the sulfides in the clay also need oxygen to convert to sulfur dioxide and gas out?

I've had sulfide issues like you've shown, on a previously used (lizella) body, with the special slow bisque, when packed tight, stacked, with the peeps in.  I've now switch to 2 peeps out til 1800 and have had a lot better results.  But I also haven't gone back to that specific clay body.

Thanks for the write up

 

Liam: not addressed, but a good point. Several other  universal kiln principles still apply.. A dense bisq load as you pointed out would benefit from a pulled bunge. Red body clay is often used for very large platters and yard planters. Potter posted a pic of a 56lb bowl awhile back; with 3/4" walls. A case were sand should be under the foot for movement; and the ramp rate slowed to 80F an hour or so. An extended hold at 1800F would also be advisable. Several studies show it takes ambient kiln temperature up to 30 minutes to penetrate a 1/2" clay wall. 

Tom

[jrgpots]

I remember hearing of a mine that hit a large seam of mica, iron disulfide.    The mica dust aerosolized.  Before it could be scrubbed from the air, the mine went up like a powder keg.

A few years ago I ground up some mica, placed it in a crucible and set it on fire.  The sulfur dioxide burned out, leaving black iron oxide.  It was a cool experiment.   Anyway,  I'm curious how much can you add to glaze or clay body before you have a problem?   

[Babs]

My son was cleaning sand out his bore.

It had silver flakes through it. Really attractive. It was like the stuff found sheeted in granite sometimes. What wouls that do if added to clay?

[glazenerd]

4 hours ago, jrgpots said:

 

A few years ago I ground up some mica, placed it in a crucible and set it on fire.  The sulfur dioxide burned out, leaving black iron oxide.  It was a cool experiment.   Anyway,  I'm curious how much can you add to glaze or clay body before you have a problem?   

Commercial iron bearing clays run 5 to 8.4% iron content; less after being blended. Your experiment illustrates what causes carbon coring. Starve the oxygen during burnout and sulfur dioxide gas reduces the iron and causes carbon coring (black coring.) Supply plenty of oxygen during burn out phase; and it off gases as carbon trioxide with no adverse effects.

4 hours ago, Babs said:

It had silver flakes through it. Really attractive. It was like the stuff found sheeted in granite sometimes. What wouls that do if added to clay?

The large opaque, clear, or silver crystals found in granite is crystalline silica. In the States; certain  granite is ground to make silica.

[jrgpots]

12 hours ago, Babs said:

My son was cleaning sand out his bore.

It had silver flakes through it. Really attractive. It was like the stuff found sheeted in granite sometimes. What wouls that do if added to clay?

At cone 6 it would not melt.  It could give you a speckled clay body.  John Baymore and Biglou have a thread about granite chunks in the clay body as well as in glaze.  They were firing to cone 10-12 over days.  Jonh Baymore still has a granite glaze on his gallery.

cone 10 suggestion  (given to me from John Baymore)

granite dust      80

whiting                20

(He also suggested granite 70 and ash 30)

 

If you want to get to cone 6 , I would start  with the below recipe to begin testing.

granite dust      67

whiting                17

3124 frit             16

 

Jed

[Babs]

2 hours ago, jrgpots said:

At cone 6 it would not melt.  It could give you a speckled clay body.  John Baymore and Biglou have a thread about granite chunks in the clay body as well as in glaze.  They were firing to cone 10-12 over days.  Jonh Baymore still has a granite glaze on his gallery.

cone 10 suggestion  (given to me from John Baymore)

granite dust      80

whiting                20

(He also suggested granite 70 and ash 30)

 

If you want to get to cone 6 , I would start  with the below recipe to begin testing.

granite dust      67

whiting                17

3124 frit             16

 

Jed

Jed these are flakes like thin foil.

I have picked up flakey sheets from granite areas .

I will get a photo.

These are coming up in fine white sand from an aquifer..

Maybe I've struck silver :-))))

[liambesaw]

31 minutes ago, Babs said:

Jed these are flakes like thin foil.

I have picked up flakey sheets from granite areas .

I will get a photo.

These are coming up in fine white sand from an aquifer..

Maybe I've struck silver :-))))

Silver doesn't occur as metal you can find like gold.  It oxidizes so it presents as a black mineral.

[jrgpots]

It could be mica or a borate sheet like leucite.  Can you put some in a crucible and grind it?..  If it is a borate it will easily grind to a white powder which would melt and bubble if you heated it with a torch.  If it is black when it is ground, there is a good chance it is mica.  Don't burn it.  A picture would help.

 

Agree with it not being silver.

jed

[Babs]

Yes and thanks Liam .Tongues in cheeks can't be seen in these forums......

I will get some of the sand and sieve out the flakes to roll a pot in before firing.

Pretty sparkly.

Lots of interesting geology here.

AND planes buzz and looking for lithium bearing areas .......

[BonnieBee]

I ordered mica clay just to test my skills with forming clay bodies. I tried it with the wheel (I know it is not recommended), and it was difficult, even with slower speed. It is very moist clay, and I have seen videos of wheel-throwing with it. What is the advice with using this beautiful clay?

[Rae Reich]

@Babs, in olden days, sheets of mica were employed as small windowpanes. Probably not  as expensive as sheet glass, considering it's a found resource, ready to split into thin panes. Also lampshades, as recently as the 1920's and '30's, a mellow amber tone (Frank Lloyd Wright?). Not sure how readily it was available, but it was commercial.  I think it has been used for oven peep holes, since it could  withstand more heat than glass.

[Babs]

Stuff I pucked up from seams in granite area is clear and one can easily divide it into transparent pieces.

Interesting read Rae.0