[Clayart] Aluminum foil saggar question
Robert Smith
chloride, letting them dry between coats. After the third coat, perhaps I
sprinkle a bit of sugar on the surface. Sometimes I will wrap very thin (32
gauge) coper wire around the piece. And I might had a horsehair or two for
a bit of carbon. I also might spread some bronze wool on the pot.
The pots have been terra sigged with three coats of an OM4 terra sig,
burnished with a cloth between coats, and bisqued to ^010-08,
In the raku kiln, I place the foil pots, sometimes separately, sometimes
loosely tumble-stacked. I fire to between 1130 and 1175ºF. I remove the
pots at that temp, and slowly over 15-20 minutes remove the foil. I do dont
touch the surface and try to make sure the foil doesn't scratch the
surface. (I should point out that after the pieces are thrown on the wheel,
I never touch the surface without a latex glove: I trim with a glove, I
terra sig and burnish wearing a glove; only after the bisque do I touch the
surface.)
Here is the problem: one out of 8-10 of the pieces begins to slough off the
colorant - not the terra sig, but the bright reds and yellows and purples
from the firing. Not all pots, and not immediately - sometimes it is a year
later that the blistering and then flacking begins. On others of these
pieces, there is no flaking, but there is fading - from brilliant reds to a
not unattractive mustard/bright yellow.
Because of the randomness of the occurrence, I have very few ideas as to
the cause of this. I am attaching a picture of the flaking I am describing.
Any help would be appreciated.
Bob Smith
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Paul Gerhold
Paul
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Taylor Hendrix
When you put that much ferric chloride on a pot some salt residue is bound
to remain. The bright but delicate reds is just an iron oxide dusting and
not fired to the pot. I could just dust that stuff off.
My theory is that when you pull the pots, the gaseous constituents of the
ferric chloride, water vapor, and whatever reaction products happen to have
formed begin to condense. If they didn't find themselves trapped on the
terra sig layer, they just settled on the surface. I began opening up the
foil as soon as I could, being careful not to get a snout full of the
powerful acid vapor that will escape.
Still, some of the iron colors are fugitive in that they are just dust
clinging to the surface. All you can do is fix it with something or remove
it. I was never willing to spray something on my pots that would fix the
oxide powder. Seemed to take the life out of the pot.
Let's not forget all those chlorine ions you bathed your pots with.
Somehow, salts will form and with that the well known hydration pops we
saggar folks call saggar acne. Again, you could try opening up the foil as
soon as you can to release the vapors before the condense on or under the
surface of your pots.
Others?
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Robert Smith
Thanks for your response. Your analysis is close to the one I had
developed. Another factor, as asked by another clayart responder, was where
these pieces were. The most severe peeling came from pieces that had been
shipped to pretty humid locations - MN, GA, and FL. And I wondered about
any connection there. Here is Colorado (low humidity) I have far less, if
any of the peeling, but do get some fading.
I did get the luscious reds from the red iron oxide (indeed in a fuzzy
state right on the surface, easily rubbed or even blown off the pot) and
yellows (a little cooler in the kiln) and some nice mauve/purples, which I
took to be developed in a temperature range between the yellow and the red.
That it doesn't happen to all the pieces is problematic, but I suppose
could be caused in part, by how quickly the foil was opened, as you
suggested. When you did open your foil immediately did that seem to make a
difference?
Also, the term saggar acne is new to me. What I experienced on the surface
was increasing blistering (not unlike sunburn blistering) and then sheets
as small as a pencil eraser and as large as a silver dollar or even bigger,
would peel and then flake off. Does the acne refer to the blistering?
When the pots were cool out of the foil, I did spray a nasty (toluene,
acetone, xylene) fixative on them. It held the RIO in place, but possibly
also trapped in any residual moisture above the terra sig/below the ferric.
What do you think of that theory? If I had waited a day or so, there might
have been some fading of the colors, but possibly less residual moisture
in/on the pot - maybe? After the spray sealant, the next day, I would wipe
on a tile sealer from Home Depot, possibly further trapping ambient
humidity. .
It DOES make sense that the concentration of ferric CHLORIDE would be the
culprit; when I used to fire to a higher temp, maybe 1400ºF, and then run
the fired pieces under water after they had cooled, this problem did not
exist. Of course I also didn't get the colors that I now do and have come
to love.
But I have already spent nearly $500 in returned money to people who bought
these flaked pots and returned them to me. In both cases, I had the pots
for nearly a year (over a year for one of them), had shipped the pieces to
the humid areas, and was immediately notified of their self-destruction.
I would hate to quit this firing method. Though I prefer clay saggar to
foil saggar, the clay saggar pieces are not nearly so dramatic.
BTW, if you are instagram, you can see my work at bobsmithraku. I am
attaching a recent pot, still no blisters, but some less red now, and more
yellow than when I shot the piece 6 weeks ago.
Thank you so much for your time and thoughts. I have a call into a
materials chemist to see if she thinks there may be ways to prevent/avoid
the problem.
Thanks again,
Bob
On Fri, Sep 23, 2016 at 10:59 AM, Taylor Hendrix <wirer...@gmail.com>
wrote:
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Robert Smith
Thanks for your response.
The most severe problems have occured when the pieces were shipped from a
low humidty place (CO or AZ) to a high humidity place (MN, GA and FL) I am
coming to believe that this is a factor.
And see below a copy of an email I sent to another clayart responder, which
might also answer other questions you could have - and whose message to me
might have begun to answer some of my questions.
Thanks again,
Bob
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Robert Smith
Did the email to Taylor Hendrix appear on the email I just sent you? I can
resend if necessary.
Bob
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Paul Gerhold
My guess is you really have two separate problems. One problem being peeling and the other being fading. You probably are not going to want to hear this but there is most probably not going to be any solution to the fading problem. You can slow it down for years but eventually no matter what coatings you use it is going to occur. Back in the old days of those beautiful multicolored Matt Raku glazes people tried almost every sealer available and no matter what the colors eventually faded. Same thing happened with the low fired metallic Raku copper glazes. Try any sealant you want and then put the piece in bright hot sun for a year or so and watch.
As for the peeling probably a combination of slight changes in application and a really good sealer might work although I am not sure if moisture is the problem. May be just a coincidence that the most returns are from humid states since I am sure your pots are probably not outside and in Fl. at least we all use air conditioning. Also for every pot you get back or complaint there are a lot more pots that have gone bad.
My advice would be to chalk it up for the time being at least because no matter what solutions you come up with you really won't know if the work for a couple of years at best . Of course some of the really technical guys may know of way to accelerate the testing but I don't and the risk of a few years of bad pots would seem one that would be tough to take financially and reputation wise. More true if you are selling through galleries.
Paul
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sumi
It occurred to me, have you ever tried ferric sulphate? I used to use it
in pit firing.
Sumi
> It DOES make sense that the concentration of ferric CHLORIDE would be the
> culprit;
Oh, and Clayart scrubs attachments - you can't attach photos to your
posts. It's inconvenient sometimes but people are leery of attachments.
The solution is to include a link to a photo on your website.
PETER HAMER
>Bob
>It occurred to me, have you ever tried ferric sulphate? I used to use it in pit firing.
How did it work out?
H&S has stopped me trying ferric chloride, but I always understood that its unique selling point"
was its volatility -- giving nice blend effects.
If ferric sulphate is a suitable alternative maybe an even better one for "archival" purposes would
be an organic salt, which would likely leave no residue other than iron oxide.
Robert Smith
I remember the days of the fading copper mattes, and the replacement glazes
that followed, with names like No-fade Copper Penny, etc. I stayed away
from them then (the '80's) and I haven't gone back.
I fear you are right about the fading and the peeling, though I feel there
must be a connection, something like the oxidation of the coppers, only
with the salts of the ferric chloride and ambient humidity/adherence to
terra sig.
Neither one is acceptable, of course, and pulling them off the shelves in
my galleries is all I can do. That, and try not to anticipate the return of
more peeling pots down the road.
I do have a meeting with a chemist later this coming week to discuss this,
and I will see what she has to say about the problem, and the differences
between ferric chloride and sulfate in this sort of firing. I have never
used the sulfated form of iron - have you?
Thanks for your help in this matter.
Bob
On Sat, Sep 24, 2016 at 12:21 PM, Paul Gerhold <gerho...@gmail.com>
wrote:
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sumi
It seemed to contribute to the overall color in the pit, adding rust
tones. I bought it at Home Depot in the gardening section where it is
sold as a fertilizer. It comes as dry granules and I would pulverize
them in a blender. I stopped using it because I hated breathing the dust
when I opened the blender and we get so much color from salt and copper
and seaweed etc. that it seemed like we could do without it. Ferric
sulphate is what is in my iron supplements but I didn't think I wanted
to breathe it. For Bob's purpose perhaps he could make a saturated
solution with the granules and use it exactly the same way. We are
postulating that the leftover chlorine in the ferric chloride forms some
kind of salt that absorbs moisture from the air and damages his pots - I
don't know what the sulfur would do, if it might form some equally
hydrophilic compound in a porous pot. It is all hypothetical.
Sumi
sumi
told me on this forum. So now I have a question. Or, well, three questions:
Nils Lou in Mel's book says: High gas pressure causes heat to go lower
in the kiln
Bill Schran says: For a downdraft kiln if one wants to heat the bottom
more, then the damper should be opened, not closed
I've always equated closing the damper with turning up the gas pressure,
as both actions increase the amount of reduction. If I want the amount
of reduction to stay steady, I increase the gas pressure and open the
damper at the same time.
So how does higher gas pressure push the heat the same direction as
opening the damper? Shouldn't those two actions have opposite effects on
where the heat goes?
And is this equally true of atmospheric and forced-air burners?
So the next question is, when any kiln is first heating up, with
moderate gas pressure and fully open dampers, I've always assumed the
top is naturally going to be hotter than the bottom. Then when you put
it into reduction by closing the damper, the heat evens out and goes to
the bottom. That's the way any kiln I know of works, not that I've fired
very many. In that case closing the damper pushes heat to the bottom,
the opposite of what Bill said. Is that not true of all kilns? In most
downdraft kilns, when it heats up with the damper fully open, does the
bottom get hotter than the top? Or does the principle Bill enunciated
apply only after the kiln is in a state of reduction?
Sumi
William Schran
You may find this information about kiln design and firing principles,
written by Fred Olson, helpful in understanding the operation of a gas
fired kiln:
https://ceramicartsdaily.org/wp-content/uploads/2010/05/gaskilnsfreemium.pd
f
Bill
Bill Schran
www.creativecreekartisans.com
wsc...@cox.net
On 9/25/16, 7:22 PM, "sumi" <su...@herwheel.com> wrote:
>Okay, I've read Mel's book and tried to absorb what various people have
>told me on this forum. So now I have a question. Or, well, three
>questions:
>
>Nils Lou in Mel's book says: High gas pressure causes heat to go lower
Vince Pitelka
The statement "high gas pressure causes the heat to go lower in the kiln" is pretty vague without any information about damper and primary and secondary air. I think that what Nils is making a generalization that higher gas pressure in a venturi burner or power burner usually results in a shorter flame inside the kiln, which will help concentrate heat at the bottom closer to the burner ports.
For best heatwork and even heat in a downdraft kiln, we generally regulate the damper for slight back-pressure which helps the heat circulate up to the top of the kiln and back down again. Pushing the damper in a bit decreases the intake of secondary air and thus lengthens the flame and concentrates more heat in the upper part of the kiln and decreases the degree to which the heat is pulled back down towards the damper. Opening the damper a bit pulls the heat more directly towards the flue opening and thus the heat doesn't make it to the upper part of the kiln and concentrates more in the lower part. This is all pretty common sense, and it has worked that way on every downdraft kiln I have ever fired.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpit...@dtccom.net
https://sites.tntech.edu/wpitelka/
sumi
and Bill, and Ron, and Mel and everybody else, thanks so much for
patiently answering all my questions about firing. I know you are all
very busy and it is very generous of you to spend any time at all on me.
I have the kiln loaded with a bunch of test pots, a few refires, and a
bunch of unsuspecting students' pots so I hope it goes well! I'll be
firing tomorrow and at least I feel armed with a little more
understanding than I was two weeks ago. I will experiment a little bit
with the damper and gas pressure although hopefully not enough to screw
up those student pots. Seems like I fired for all those years in a very
non-standard kiln that worked somewhat backwards. Or I just never played
with it enough. It is candling on 1/2" of pressure right now.
Sumi
jonathan byler
I’ve found there is a sweet spot for firing our kilns, where opening the damper too much causes problems, and closing too much causes the top to get too hot early in the firing. this sweet spot is proper for the amount of gas and air that I’m letting in. The big problem with all this is that changing one variable often makes more than one thing happen in the kiln. Increasing gas pressure increases back pressure all things being equal. there’s a point where increasing gas pressure doesn’t make a noticeable difference in back pressure because you’re still working within the constraints of the clue opening and chimney draw of your set up. but when you pass this critical point, it becomes very obvious that more gas is helping create more back pressure.
moving the damper in or out creates more back pressure or lessens it. closing causes less secondary air to be pulled in past the burners, eventually leading to a reduction atmosphere in the kiln. opening too wide can cause oxidation atmosphere in the kiln, as there is more air entering than is needed for complete combustion. The problems we have because of this are compounded usually by incomplete mixing of gasses in the burners, which the target brick helps to remedy.
Changing the primary air affects mixing of fuel gas and air, affecting flame length, and where the hottest part of the flame is in the kiln. more air than needed for complete combustion gives a shorter flame, less gives a longer flame that tends to go up in the kiln if there is not a lot of draught from the chimney. if you have a lot of draught in the chimney, a long reducing flame can be pulled down and straight towards the flue, not giving it time to give its heat to the top part of the kiln. I really like power burners because of this, since I can really use the air to push heat around in the kiln.
It’s a complicated dance with even just the variables of primary air control, damper control which controls both the venturi effect of the flue, and also the secondary air coming in, and gas pressure. add in target bricks, bag walls, kiln shape, and stack pattern, and it gets even messier.
The main thing to remember is that the moment the gasses enter the kiln and combust, they give off the heat they create to the ware, and the kiln walls and the furniture. The higher the differential temperature between the flame and the ware, the more of that heat gets absorbed by it. But there is also a time element involved. The longer the flame is in contact with the ware, the more of its heat can be absorbed by the ware. so velocity comes into play. the ideal conditions for flame velocity and combustion change throughout the firing as the ware and the kiln gets hotter, so all designs are a bit of a compromise. Think about the path the heat is taking as you stack the kiln, and as you adjust the burners. if there is nowhere for flames to go easily, heat is not going to go there. if the top is getting too hot, stack the bottom looser and the top tighter. if the front is getting too hot, move the target brick an inch at a time towards the back. if the bottom is not getting hot enough in the front, stack loosely there, and make sure there is a path from there towards the flue under the shelves.
It’s really a shame we can’t have a glass walled kiln, so that we can see where the flames are actually going, as it would make this whole process of adjustments a whole lot easier.
jonathan byler
our pilot burners are powerful enough to bring the kiln up to about 5-600F overnight. because of the very low pressure in the kiln, and low velocity of the flame, all the heat tends to go to the top of the kiln and get released early in the firing if no steps are taken to correct this. I’ve found that once that heat differential has started to do its work, it can be extremely hard to remedy. My fix is to nip it in the bud from the very beginning. I leave the bottom spy plug open overnight, and close the damper leaving only 1/2” opening at the very most. I run the pilot burners, and set the air to give just enough flow to help create turbulence in the kiln and spread the heat around. If I’m not feeling warm/hot air coming out of the bottom spy hole, I know that I need a little more air. Too much air disturbs the flame on the atmospheric pilot burners, so there is a spot where you can go to far and cause problems with the pilot flame. This combination of back pressure from the flue, and the extra air from the blowers seems to keep things stirred up well, and instead of being 500F at the top and 150F on the bottom, I get much more even temperatures when I come in the morning to start the main burners. firings for me are much shorter and much more even when they start even.
> On Sep 25, 2016, at 6:22 PM, sumi <su...@herwheel.com> wrote:
>
PETER HAMER
You might like to show this paper to your chemist friend
http://tinyurl.com/gvo7lhn
It looks like ferric chloride has some subtle advantages in this application area.
PS
I came across the paper when trying to find papers on spray pyrolysis of iron salts of short-chain fatty acids.
No success with iron salts, but a fair number of hits for salts of other metals.
If your friend knows of any easy way to calculate the same thermodynamic properties for, say, ferric
and ferrous acetate I'd like to know.
----Original message----
From : raku...@gmail.com
Date : 25/09/2016 - 23:19 (BST)
To : cla...@lists.clayartworld.com
Subject : Re: [Clayart] Aluminum foil saggar question
sumi
Thanks for taking the time to write all that out, including your
previous message. It is beginning to make sense. I should print it out
and take it with me! Our pilot doesn't warm the kiln at all so I've
always candled on 1" with the door cracked just a tad and the peepholes
open. I've always had the damper wide open, too, just to get a draft
going. Then it's 1200 or so in the morning, cooler on the bottom. Last
night I had it at 1/2", thinking I'd have less temperature differential,
and it was 900 on top, 750 on the bottom, not bad I thought. But what I
discovered in the next couple of hours is that I must have blocked off
the flue channel too much and the kiln started reducing all by itself at
1500 with the damper fully open. So I turned it off and I'll reconfigure
it a bit on the bottom and try again. The shelves are on bricks above
the flue, but several firings ago I tried arranging several extra bricks
in that space to slow the flame down - now with other changes I may have
baffled the flame too much! I tried to create a flue channel so the
flames would have to go over the load and down the front to get out. Or
I'll find a pot or something actually blocking the flue or inside the
chimney. I may try your trick of closing the damper part way when it is
candling overnight, I have done that in the past and it does help even
it out overnight.
Fortunately my students have been very tolerant so far. I hope I didn't
spoil too many of their pots this time.
Sumi
Paul Gerhold
Paul
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