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Fertilizers For Pit Firing

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I'm wondering what component in fertilizer contributes color to a pit fired ceramic piece.  Some fertilizers are more expensive than others, so I'm thinking that if I knew which component created color in a pit firing, I could price shop fertilizers without running the risk of buying a cheap fertilizer that is missing the agent needed.  Lots of folks on the Forum have mentioned Miracle Grow, but it's relatively expensive.  I'm hoping to find a cheaper alternative....

Jayne

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In another thread in the clay and glaze forum talking about saggar firing, copper nitrate, a root killer was mentioned to be less costly than Miracle Grow. Many nurseries will carry it in large bags.

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Copper sulfate is an active ingredient in root killers, it may be the only ingredient other than inert filler.

 

I am not sure that copper nitrate is what you want...quoted from an article "Cupric nitrate is a strong oxidizer which can cause burning and explosive if heated, rubbed or impacted with carbon powder, sulfur or other combustible materials"
 

John
 

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Copper sulfate is an active ingredient in root killers, it may be the only ingredient other than inert filler.

 

I am not sure that copper nitrate is what you want...quoted from an article "Cupric nitrate is a strong oxidizer which can cause burning and explosive if heated, rubbed or impacted with carbon powder, sulfur or other combustible materials"

 

John

 

Thanks John for the correction.

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There is no copper sulfate in plant fertilizers. they contain mixed phosphates, nitrates, and maybe some iron. If you want copper sulphate, then you should specify "root killer". This is almost pure copper sulphate crystals. This (and ferric chloride) is what most people use in pit fires or aluminum foil sagger fires.

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Thanks, all. I had never heard of a dry root killer until I read these posts and Googled it.  I found copper sulfate crystals as well as powder, but I only found liquid versions of ferric chloride.  Surely you don't pour liquid ferric chloride on pottery?  Are either of these available at big box hardware stores - or only available online?

Jayne

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Yes you do pour liquid ferric chloride on pottery, or brush it or dab it on with a sponge. It is corrosive stuff so you have to be careful. The root killer you can get at Home Depot or Lowes. The ferric chloride used to be sold by Radio Shack to make electronic PC boards, but I don't think they have it any more.

 

I make my own with muriatic acid and steel wool (Home Depot). Just put a pint of water in a quart jar and add half a pint of muriatic acid SLOWLY then drop a couple of pads of steel wool in and let it sit. Keep throwing pads in till no more dissolve. the yellow liquid you get is ferric chloride.

 

I don't recommend you do this unless you have some understanding of the chemicals and wear gloves and safety glasses. There is lot's of info on the internet if you want to pursue it.

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Jayne,

 

You can get ferric chloride solution (relatively locally in most places in the US) from Radio Shack....... it is used for etching printed citrcuit boards.

 

best,

 

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

 

EDIT:  Hummmm.... Bob says above they don't carry it anymore. Oops.. sorry.  Haven't bought it it a while.

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> Bob Coyle: I don't recommend you do this unless you have some understanding of the chemicals and wear gloves and safety glasses. There is lot's of info on the internet if you want to pursue it.

... and take adequate precautions against the corrosive fumes. I've read of somebody having permanent vocal-chord damage.

This certainly seems to be something that should be done in private, or only with knowledgeable consenting adults.

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Thanks Bob, Peter and John.  I'll certainly read up on making ferric chloride before I attempt it!  Can you tell me what color(s) ferric chloride contributes in a pit-fire?  For that matter, what effect does copper sulfate have? I know that personal experimentation is the best way to answer these questions, but pit firing is so labor intensive that I'd rather not spend half a day firing just to create a test tile or two...or even worse, to experiment on a sculpture that I've put lots of time into!  

 

I searched all three guys' galleries hoping to see an example of a pit-fired pot using those chemicals, but no joy. (It was a pure pleasure searching the galleries, though -- John's yakishime with youhen charcoal finish is breathtaking and Bob's electroformed pots are truly fascinating).   Nonetheless, I'm wondering if anyone knows of an image of pit-fired pots using those chemicals.

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What you get depends on the firing conditions. With ferric chloride you will pretty much get a yellow or orange color on white clay. Copper sulphate will give you a black or grey or (if you are lucky) a reddish blush under reducing conditions.

 

Do a google search under pit fired pottery and you will get an idea of what it looks like. There are also a few vidios on Youtube to get you started.

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What you get depends on the firing conditions. With ferric chloride you will pretty much get a yellow or orange color on white clay. Copper sulphate will give you a black or grey or (if you are lucky) a reddish blush under reducing conditions.

 

Do a google search under pit fired pottery and you will get an idea of what it looks like. There are also a few vidios on Youtube to get you started.

Thanks, Bob.  Now I'll know what I'm looking at when I visit some pit firing websites.

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If you want something simpler and safer, just use salt. Make a super strong saltwater solution using rock salt in a 5 gallon bucket and soak wood chips in it for a day. Let the chips dry out on the driveway and use them in your pit. Lots of nice reds, pinks, oranges and yellows without worrying about chemical burns or blowing up the studio.

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If you want something simpler and safer, just use salt. Make a super strong saltwater solution using rock salt in a 5 gallon bucket and soak wood chips in it for a day. Let the chips dry out on the driveway and use them in your pit. Lots of nice reds, pinks, oranges and yellows without worrying about chemical burns or blowing up the studio.

Holy Moly, that sounds exciting!!!!  Thanks!

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Hi Jayne (and others),

 

I found this excerpt from a Sumi von Dassow video to be very helpful. She gets amazing results!

 

http://ceramicartsdaily.org/firing-techniques/pit-firing-video-a-guide-to-gathering-fuels-for-the-best-results-in-a-pit-firing/

 

Happy firing,

 

Jean

BEAUTIFUL pot, Jean!  And thanks sooo much for this video!  I've searched for something this specific for a long time. I can't believe I now have access to such a wealth of information! Thanks again! 

Jayne

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Thanks for the compliment - I'm wearing my "aw, shucks" face....

 

If you haven't already seen it, the book "Alternative Kilns & Firing Techniques" by James C. Watkins & Paul Andrew Wandless has a lot of information, too, especially if you want to work with ferric chloride. Plus, if you think you may want to experiment with trash can/barrel firing, here is an interesting post from Bonnie Staffel on a clay art thread that I enjoyed reading:

 

http://www.potters.org/subject90543.htm

 

I promise I'll stop now!

 

Jean

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Thanks for the compliment - I'm wearing my "aw, shucks" face....

 

If you haven't already seen it, the book "Alternative Kilns & Firing Techniques" by James C. Watkins & Paul Andrew Wandless has a lot of information, too, especially if you want to work with ferric chloride. Plus, if you think you may want to experiment with trash can/barrel firing, here is an interesting post from Bonnie Staffel on a clay art thread that I enjoyed reading:

 

http://www.potters.org/subject90543.htm

 

I promise I'll stop now!

 

Jean

Jean, Don't stop!!  Seriously, this is such useful information.  I've read everything I could find & watched youtube videos, but the information is usually very general.  And it's so helpful to know the "why's" of something.  (kind of like giving a fish to a hungry man vs teaching him to fish). This article makes the point that the lower temperatures in sawdust firing can't fume the salt to produce red coloration, which helps me to understand why my trashcan fires haven't worked as I hoped.  I can't figure why Bonnie is putting pet litter sprinkled with copper and salt, as a bottom layer beneath the pots, though.  Any thoughts on that?  Also, regarding Summi's video, I'm wondering about that blower system of Summi's.  I don't think I've seen metal pipes like that, perforated all over with holes.  She says that they only last through 3 or 4 burnings, so they clearly aren't made for this kind of use.  I  wonder where would a person find the kind of metal pipes that she uses?  And I'm guessing that's just a leaf blower or some such thing to provide the air?  Since you created that beautiful pot, would you mind sharing your technique?

Thanks, Jayne

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Jayne, My "technique" for pit firing? Simple: drive to Colorado and sign up for a pit firing session with Sumi von Dassow at Washington Heights Arts Center in Lakewood, CO. I am embarrassed to say that as I was mainly interested in techniques that I could bring to trash can/sawdust firing, I didn't pay very much attention to the actual pit construction. I've tried to find photos from the trip for you but it appears that my computer has eaten them. I SEEM to remember that the pipes had slashes cut in them rather than holes, but I could be mistaken. And the blower looked very much like a large leaf blower, but it would have to have been one with an on/off switch, not a trigger-type start. What I did learn and remember that has helped my firings is to stop using porcelain (I switched to Laguna cone 5 B-mix), lower my bisque temp to cone 010, and add copper carbonate rather than just copper sulfate (found in MiracleGro). She recommends the use of terra sigillata, but I prefer just to burnish my pieces.

 

As to why Bonnie Staffel would use Pet Litter, I am just guessing that she is referring not to cat litter but to the wood shavings used as bedding for many pets. It's larger than sawdust, won't pack as densely, and will therefore fire hotter and quicker, vaporizing your chemicals better without just smoldering and producing mainly shades of black and gray like fine sawdust. I go to a feed and grain store here in Kansas City and buy aspen wood shavings. It may be my imagination but I think I get better colors with the aspen rather than cedar shavings.

 

My apologies if I've gone on too long, but as someone who has lurked on this forum for well over a year, it's nice to be able to share, if only a little bit.

 

Jean

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Bottom line is this...as always. Ceramics is 90% art 10% science, even for a chemist like me. Read up on what you want to do. Check out references on the internet then experiment with what you want to have happen. Come up with YOUR method with YOUR chemicals with YOUR process and tweak it till it gives you what you want.Half of what you read and see about pit firing has nothing to do with chemical fact. It is folklore. Seaweed, dung, bark, oak leaves all will give a certain effect SOMETIMES or maybe NOT.

 

The methods people use and the variables are too diverse to really duplicate from one situation to the next. I guarantee your firing will be different from any other you have read about or seen.

 

Controlled kiln firings with commercial glazes show variations from one kiln to the next. No way is any pit firing going to be reproducible from one to another. You can use chemicals and firing conditions to push the pit fire result one way or another but don't rely on other persons experience to tell you what the result will be. Just go for it and be aware of what works and doesn't work for you and take joy in the fact that you are participating in a tradition that spans at least 8000 years or more.

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Jayne, My "technique" for pit firing? Simple: drive to Colorado and sign up for a pit firing session with Sumi von Dassow at Washington Heights Arts Center in Lakewood, CO. I am embarrassed to say that as I was mainly interested in techniques that I could bring to trash can/sawdust firing, I didn't pay very much attention to the actual pit construction. I've tried to find photos from the trip for you but it appears that my computer has eaten them. I SEEM to remember that the pipes had slashes cut in them rather than holes, but I could be mistaken. And the blower looked very much like a large leaf blower, but it would have to have been one with an on/off switch, not a trigger-type start. What I did learn and remember that has helped my firings is to stop using porcelain (I switched to Laguna cone 5 B-mix), lower my bisque temp to cone 010, and add copper carbonate rather than just copper sulfate (found in MiracleGro). She recommends the use of terra sigillata, but I prefer just to burnish my pieces.

 

As to why Bonnie Staffel would use Pet Litter, I am just guessing that she is referring not to cat litter but to the wood shavings used as bedding for many pets. It's larger than sawdust, won't pack as densely, and will therefore fire hotter and quicker, vaporizing your chemicals better without just smoldering and producing mainly shades of black and gray like fine sawdust. I go to a feed and grain store here in Kansas City and buy aspen wood shavings. It may be my imagination but I think I get better colors with the aspen rather than cedar shavings.

 

My apologies if I've gone on too long, but as someone who has lurked on this forum for well over a year, it's nice to be able to share, if only a little bit.

 

Jean

Jean, thanks for sharing your experiences with me.  I have my feelers out for a workshop a bit closer to SC, although I'd love a workshop with Summi!  Long before I had heard of her, I was visiting Taos, NM, and found and bought a decoratively fired sculpture of a woman holding a pot.  I asked the gallery owner for the artist's name, and the gallery owner went one better -- she called Summi while I waited and asked Summi to email artist info to me.  Later on when I became interested in clay as a maker rather than just a buyer, I was thrilled to find her name in reference books and articles. 

 

Y'know, about that pet litter -- I figured it meant shavings, too.  So to test my assumption I googled "pet litter" -- and all I found were references to cat litter!!!  But I'm sure you're right!!

 

Jayne

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Bottom line is this...as always. Ceramics is 90% art 10% science, even for a chemist like me. Read up on what you want to do. Check out references on the internet then experiment with what you want to have happen. Come up with YOUR method with YOUR chemicals with YOUR process and tweak it till it gives you what you want.Half of what you read and see about pit firing has nothing to do with chemical fact. It is folklore. Seaweed, dung, bark, oak leaves all will give a certain effect SOMETIMES or maybe NOT.

 

The methods people use and the variables are too diverse to really duplicate from one situation to the next. I guarantee your firing will be different from any other you have read about or seen.

 

Controlled kiln firings with commercial glazes show variations from one kiln to the next. No way is any pit firing going to be reproducible from one to another. You can use chemicals and firing conditions to push the pit fire result one way or another but don't rely on other persons experience to tell you what the result will be. Just go for it and be aware of what works and doesn't work for you and take joy in the fact that you are participating in a tradition that spans at least 8000 years or more.

Bob, thanks for the advice; I know it is heartfelt.  But the reason I'm asking for help is not because I don't want to bother with experimenting or researching -- I've done a great deal of both.  I've done a number of trash can firings and I've been present when my Native American husband fired his pots on the ground in the manner of his tribe, whose pottery tradition is believed to be an unbroken 5-6,000 year old tradition.  I know first-hand how varied the results can be, but I can't help believing that having some grasp of the chemistry involved will help to reduce the incidence of specific undesirable results.  And those are the facts and specifics that I've had a hard time finding, despite having read, viewed videos, and discussed pit firing & trash can firing for several years now.  Information like "extreme reduction = solid black pots" or "a fire that isn't hot enough = greyed and muddy pots instead of pots with bold carbon markings" can help us to diagnose the problems we're having instead of randomly altering different aspects of the firing.  If you get certain results without understanding why you got them, it's hard to know which aspects of the process to tweak. Particularly frustrating is watching my husband use the same materials that his grandmother and great-great grandmother used, yet getting the same results only occasionally.   Thanks to folks on the Forum, I now know how to avoid some of the unsuccessful results he has gotten in his coil-built pots and I have gotten in my sculptures. 

 

Jayne

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You can check out Peipenberg video on saggar firing, He mixes salt and copper carbonate in layers of newspaper, wood and sawdust.

He fires in an oil drum with a few holes to help the burn along. Also Bet Thomas has some interesting work in the Alternative Firing book.

 

Marcia

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