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Getting The Perfect Gloss From Terra Sig.

terra sigillata improving earthenware

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#1 Tyler Miller

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Posted 17 June 2014 - 11:26 AM

Terra sigillata is why I got into ceramics.  I'm a fan of classical Greek and Roman art and learning to throw pots "like them" has been a goal of mine ever since I began playing with clay.  But I'm bad at the classical style.  "Athenian Vase Construction," by Toby Schreiber has been a great help in learning technique.

 

But it's also really hard, especially the gloss.  On ancient examples, it's like a glaze, the level of shine is so high.  Check out this kylix, it's perfection: http://webapps.fitzm....php?oid=166270

 

I'd love to know how you guys approach pre-application surface treatment, application of the terra sig., and even firing (max cone range?) to get the glossiest finishes possible.  I've got some articles from CAD and elsewhere, but I still can't seem to make it work that well.

 

 

 



#2 Wyndham

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Posted 17 June 2014 - 11:38 AM

Here's a guess, based on what the early pioneers did for dirt floors, Milk.

After the floor was compacted, at which point I'm not sure, they poured milk from the cows(goats, sheep ?)onto the floor and polished them( don't know this part).

In another thread, this is mentioned for sealing earthenware pieces(after firing).

The oil and protein of milk might be what did it

I think this might be a path you might look into.

Wyndham



#3 Tyler Miller

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Posted 17 June 2014 - 03:05 PM

Cool!  I'll bet your right that it's the oil and milk protein.

 

I'll definitely give it a try.



#4 Min

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Posted 17 June 2014 - 03:26 PM

I have a friend from Hungary who used rendered elk fat when she lived there. Now she buys suet and renders that down to rub onto the fired pots. Pots are shiny but not overly so. A bit of an ick factor when I found out what she used. :blink:  (her pots are decorative only)



#5 neilestrick

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Posted 17 June 2014 - 04:14 PM

I had a studio mate a long time ago who did nice glossy terra sig pots. He would put on 3 layers (if I remember right) of sig, burnishing with a stone between layers. The last layer was burnished with lard before firing. There was some historical reference for using the lard, and it lubricated the burnishing process. The firing was quite low, like maybe 1650F, to prevent the polish from burning off. The fired pots he then waxed and buffed, although they were quite glossy before waxing.


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#6 Bob Coyle

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Posted 17 June 2014 - 07:50 PM

The pueblo potters out this way  use a little fat and burnish the heck out of the pot till it has an almost mirror like finish. Then they do a reduction pit fire. seldom gets higher then maybe 1400F from what I read from people who inserted thermocouples into the pit. Ends up looking pretty much like your picture.



#7 Colby Charpentier

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Posted 17 June 2014 - 07:55 PM

We use mineral oil in the community studio when burnishing pots for pit fire. Might be a good substitution for pig fat, milk or some of that other stuff.



#8 Tyler Miller

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Posted 17 June 2014 - 07:57 PM

Thanks for the help guys.  I'll give the lubricated burnishing method a try. :)



#9 Venicemud

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Posted 18 June 2014 - 09:17 AM

Years ago I took a workshop with Juan Quezada in Mata Ortiz and there we burnished with baby oil.  I asked Juan why he used baby oil and he said (via a translater) because it smells better.



#10 JLowes

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Posted 18 June 2014 - 09:26 AM

Although I am quite certain this has no historic basis, I got nice results from the following method.  I apply three coats of terra sigillata.  I have burnished between coats and not, but always using the unprinted parts of a grocery store plastic bag.  A friend uses a favorite bit of sweatshirt.  We both get a high shine on the last coat.  Then you bisque the pot.  I have bisqued as high as cone 04 (not knowing any better), but cone 010 or 08 will keep more shine. 

 

Here comes the definitely non-historic part; wrap the pot round and round with cheap toilet tissue, keeping as close to the form as you can.  Then wrap the pot and tissue in a layer or two of aluminum foil and seal it up tight.  I fire this up to 1000 F (although paper burns at 451 F this gets it going better in my opinion), outside in a raku kiln (it can get smoky), and shut down the kiln, remove the pot, and let it sit and burn until the paper is exhausted and cool.

 

I only found this picture as an example, but it shows the color and shine.

 

John

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#11 Bob Coyle

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Posted 18 June 2014 - 11:15 AM

Here is a vid of Maria at work. very basic technique but effective.

https://www.youtube....UGm87DE0k#t=710



#12 Tyler Miller

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Posted 19 June 2014 - 11:04 AM

Bob, thanks for tracking down that wonderful video!  I watched it all the way through last night and enjoyed it for a lot more than just the burnishing technique.  



#13 Tyler Miller

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Posted 27 June 2014 - 03:23 PM

Attached File  terrasigmystery.pdf   249.08KB   24 downloads

 

I found this photo in Athenian Vase Construction I thought I'd share.  I formatted it as a pdf because it gives better detail for its file size.  If this is a problem, let me know and I'll shrink the phot and repost as a jpeg or rescan at greater detail and crop.  It's of a kylix foot that broke off, revealing that the potter had used terra sig to join the stem to the bowl instead of conventional slip.  The slip is glossy and black from reduction, despite it not being burnished.

 

It appears there's more to Greek gloss than just burnishing.  The gloss is in the slip itself.  Puzzling, eh?



#14 Marcia Selsor

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Posted 27 June 2014 - 04:06 PM

We did terra sig orange and black at the potters Council tour in Tuscany. We did not use any fat, stones nor spoons to burnish.First we formed the pieces and smoothed the surface with a wooden rib and metal rib. We applied thin layer to dried surface and polished with a soft sponge. Did it again and if desired, one more time. Let the surface dry in between coats and polishing.Note the sponge in the first picture. That is what we used for burnishing. this was one of two hands-on workshops as part of the Potters Council Tuscany Tour IV.this past year.

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#15 Tyler Miller

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Posted 27 June 2014 - 04:40 PM

Marcia, thank you for explaining the process, those pieces are beautiful!



#16 Babs

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Posted 27 June 2014 - 06:52 PM

A piece of lambs wool rug works nicely



#17 Marcia Selsor

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Posted 28 June 2014 - 07:23 AM

The orange sig fluxes at 857 degrees C. All clays vary. This particular clay comes from the garden area at La Meridiana. two years ago we did numerous tests to nail down the exact temperature of the fluxing. That means that is won't take carbon when smoked in sawdust. The black is a more porrous slip. ball clay could be used. You have to test to get these effects.

Marcia

#18 Tyler Miller

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Posted 28 June 2014 - 08:22 AM

Ah, I see.  It occurs to me that we're talking about different things then.  I'm interested in the historical method used in particular on Greek and Roman gloss.  It's well known that the effect was achieved with one kind of slip, treated in two different ways.  

 

While I'm greatly appreciative of all the generous help all of you've offered, I did feel like there was still something missing.  I spent the night last night researching the process and I think I've just about got it down.  Testing to be sure.  In the interest of sharing, I'll post what I've learned.

 

FIrstly the black/red effect of Greek pottery comes from an oxidation, reduction, reoxidation firing schedule, with the peak temp achieved in reduction.  The black sinters enough that it will not return to orange in oxidation.  The surface treatment process as I had already known it from Toby Schreiber's book is that the pot was smoothed with a rib, then metal, then an iron-enhanced slip called miltos was added.  The potter would then burnish the pot.  The design was applied with a scraper or charcoal, then the "mystery slip" that later turns black was applied.  Schreiber describes a process of using 80% ash water to 20% iron bearing clay, stirring it vigorously, pouring off the finest particles of clay and then evaporating the slip with heat until it was the consistency of cream.  This is what I understood to be modern terra sigillata, however, in my experience they don't behave the same.  I can get a warm, shiny glow with terra sig, but I can't get the level of gloss of Greek red-figure.  It is truly like a glaze.  I welcome correction, but I've never seen a modern terra sigillata get that level of mirror polish, John Lowes' example above is the closest I've seen.  However, the broken kylix bottom I posted above also gets a truly high polish comparable to JLowes' with nothing more than basic slip-joining technique.

 

So, I spent tonight feeling a little at a loss, despite my appreciation for how much information has been offered up.  I did quite a lot of reading and I think I've come to some conclusions about the process that may be of interest.  Firstly, I have a feeling the ash water in the slip is doing more than simply levigating the particles, I think there's a level of alkali high enough to increase the level of sintering to a nearly non-porous surface.  Then, illite is repeatedly mentioned in articles as a key component of Greek gloss.  It's not universally present in modern, commercially produced clays, and I think this has somethign to do with the difference between warm, shiny glow of modern terra sig. and the high polish of unburnished Greco-Roman gloss.

 

I welcome correction on any of the conclusions I've come to, and I will be testing my theory with a oxy-red-ox firing in late July/August.  I will post results here.

 

Thanks again for all the help.  I apologize if I've caused any confusion in what I'm after.



#19 Marcia Selsor

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Posted 28 June 2014 - 09:46 AM

No confusion. I am aware of the many theories of red and black pottery from the Ancients. In this case the tradition from the first use of controlling the atmosphere to re-oxidize to get the red (orange) to turn orange again does use the similar idea of one that sinters, i..e fluxes enough to seal and the other remaining porous.
I have a good book called "Looking at Greek Vases" by Rasmussen and Spivey from Cambridge University press.1991
At any rate, one uses the kiln atmosphere and the other uses the kiln to reach the sintering temperature and then post reduction. Interesting that both give the same coloration but use different reduction process.
I don't think it is caused from milk, lard, etc. It is done with the firing process and the knowledge of the sintering temps of the slips.

Marcia

#20 Angie Days

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Posted 28 June 2014 - 09:03 PM

There are two things you have to take into account: first, the photo is retouched and the lighting design is made with perfection. 

Have you ever seen a Kylix in a Museum? The brightness is silky, not as shinny as in the photo.

Second, some natural red clays produces it’s own shine even without polishing, the brightness is lower but it’s still there. Terra sigillata has a rule: higher temperature, less luster.  Try working with a very plastic natural clay with low fire temperature and see the difference.

P.S. In the book the photo is not as shinny as it looks in your photocopied page.

some links you'll find interesting:

http://books.google....epage&q&f=false

http://www.jstor.org/stable/497642

http://iweb.tntech.e...l/terra_sig.htm

 

I just found this amazing information:

NEW EVIDENCE FOR THE NATURE OF THE ATTIC BLACK GLOSS

http://www.ims.demok...rchaeometry.pdf







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