How did the Romans make Terra Sigillata?

[ThruTraffic]

I've found very little on this topic (how to create the liquid based t-sig). If I understand correctly the Romans and Greeks didn't have/use a deflocculant in their recipes and were still able to achieve a gloss finish after firing.

I found one YouTube video in english that was created by a researcher in Europe. I 'think' I understood he was able to recreate the Roman/Greek level of gloss without a deflocculant but at great effort by manually adding clay to water (hours of doing it slowly with his fingers). His fired results certainly did look as glossy as ancient terra sig pottery.

Apparently (according to Wikipedia) a German chemist named Karl Fischer was able to recreate ancient terra sig but I'm unable to find any detailed evidence of how he achieved it.

So, who know how to create terra sigillata (the liquid mixture, not the pottery) the Roman way?

[PeterH]

8 hours ago, ThruTraffic said:

I found one YouTube video in english that was created by a researcher in Europe. I 'think' I understood he was able to recreate the Roman/Greek level of gloss without a deflocculant but at great effort by manually adding clay to water (hours of doing it slowly with his fingers). His fired results certainly did look as glossy as ancient terra sig pottery.

Can you give a reference? It would be interesting to know why he chose to mix by hand rather than mechanically, sounds a bit hair-shirt to me.

You may find parts of this video of interest, especially from about 24m to 31m. It describes the gradual realisation of the nature of the slip used in antiquity for things like Greek red-and-black and "Roman" Samian ware.
Terra Sigillata: Lost & Found by Peter Pinnell & Rhonda Willers
Long before glazes appeared, clay finishes were widely used. Potters around the Mediterranean developed surfaces employing the unique properties of clay. Once lost, Terra Sigillata was found after 1500 years. This co-lecture provides a history of this surface with an overview of today’s practices.

... of course this doesn't say how it was made. I suspect this is lost in history. I doubt deflocculation would leave identifiable traces, and it the process may well have been a trade/craft secret.

Use of deflocculants such as soda/pearl ash would have been possible. I've even seen a suggestion that a good way to find a natural source is to wait a decade or so for a major drought and then explore the dried-up beds in the local lakes. Some of them will consist of ultra-fine particles (sort of a super ball-clay*). Personally I've wondered if waste-water from the production of clay for the bodies could have been a starting point, as it would probably have contained colloidal particles.

* Supposedly the change of pH when river water enters a lake  favours the precipitation of colloidal particles, potentially leading to a very fine silt.

PS The video quotes the historic use of the term "magnetic" - obviously we would now say "electrostatic".

Edited December 12, 2022 by PeterH

[ThruTraffic]

I wanted to note the research video but so far unable to find it again; even in YT watch history. Will try again. I've seen the Pinnell/Villars video before. The last part of your response notes some good avenues for study; probably past my scientific/chemical knowledge base though. :-) Thank you.

[Kelly in AK]

Isolating fine particle sizes by allowing clay to settle in water (elutriation, I had to look that up) was known to potters historically, actively used in places where clay had to be “washed” to make it useful. Terra sigillata is an extreme extension of this principle. Natron, naturally occurring soda ash, was well known to the Romans and had many uses, including in ceramics and glass. To me, it’s a stretch to imagine Romans didn’t use a deflocculant of some kind. Looking at the artifacts, I think it’s also important to consider the impact of firing on the finished appearance. Roman terra sig appears different than Greek ware, but it may not all be due to how they made their slip.

[Callie Beller Diesel]

20 hours ago, Kelly in AK said:

To me, it’s a stretch to imagine Romans didn’t use a deflocculant of some kind.

This. 

They did so much work with plaster and concrete, much of which wasn’t re-figured out until recently. The likelihood of someone not working out a deflocculant seems very low. 

[ThruTraffic]

On 12/12/2022 at 2:04 PM, PeterH said:

Can you give a reference? It would be interesting to know why he chose to mix by hand rather than mechanically, sounds a bit hair-shirt to me.

Not sure what hair shirt means but it appeared the researcher did employ the scientific method in his work. Unfortunately that video seems to have been removed. I searched my watch history and I left a comment there and the video and my comment do not exist in my YT history. Bummer.

[Callie Beller Diesel]

@ThruTraffic some catholic priests used to wear an itchy shirt made of horsehair as penance, so they’d be forgiven for their sins. That’s the cole’s notes version, but enough for your own google search. Saying something is hair shirt is saying that it’s unnecessarily uncomfortable/painful/difficult.

[PeterH]

On 12/12/2022 at 12:50 PM, ThruTraffic said:

If I understand correctly the Romans and Greeks didn't have/use a deflocculant in their recipes and were still able to achieve a gloss finish after firing.

After some searching I found this paper. Which contains some though-provoking facts and ideas.
Recovery and Revival of Attic Vase Decoration Techniques
What can they offer archaeological research ?
https://tinyurl.com/yecn6xnf

Of course the firing (and final effect) of this "black gloss" is different from that of t-sig, but they both start by forming a "colloidal slip" (with or without the use of a deflocculant).

p114 Gives the results of  X-ray analysis of historic samples and modern "reproductions"
- This clearly shows that the originals didn't use phosphorous-based deflocculants  (many reproductions used calgol).
- The "glaze" contains much lower calcium levels than the body.

118 It is pointed out that
- The absence of phosphorous in the glaze implies that  plant-ash deflocculants weren't used.
- Although not conclusive the potassium levels suggest that potash defloculants weren't used either.

 p119  Looking for a deflocculant-free method of producing the "colloidal slip".
- Using a low-calcium clay (similar to that used in antiquity) they found that they could get acceptable (deflocculated?) results a pH>8.7 and >15°C.
- Using the same clay pre-soaked for six months they got similar (not-deflocculated?) results at pH~7 and >15°C.
... this suggests that slip production was a seasonal process, probably during the warmer spring to autumn.
... assisted by over-winter soaking of the clay.

The difference in calcium content between different clays is shown in fig2

PS I couldn't help speculating:

1. That the need for low-calcium clay was because that any soluble calcium compounds would act as deflocculants. After all soluble calcium and magnesium compounds  are used a deflocculants for glaze slips.

2. Historically they may have found suitable candidate clays simply by paddling round the local clay-pits in late spring, looking for signs of spontaneous colloidal dispersion.  Those wishing to find suitable modern commercial clays may need to look at them after a 6-months soak.

Edited January 4, 2023 by PeterH

[ThruTraffic]

1 hour ago, PeterH said:

2. Historically they may have found suitable candidate clays simply by paddling round the local clay-pits in late spring, looking for signs of spontaneous colloidal dispersion.  Those wishing to find suitable modern commercial clays may need to look at them after a 6-months soak.

Time to buy a drone? :-)

[Callie Beller Diesel]

Or just go on long walks. Makes sense.  According to my ceramic history class, a lot of Roman era potting in Europe was a seasonal, part time gig. You did it when you weren’t busy farming, and there’d be time to source and prepare materials in advance. 

[PeterH]

I find terra sigillata a confusing term, which is also often unrewarding to use in searches. Some of the issues are described in
Terra sigillata https://en.wikipedia.org/wiki/Terra_sigillata
 

The introduction is especially relevant, and I include the first paragraph of it.
Terra sigillata is a term with at least three distinct meanings: as a description of medieval medicinal earth; in archaeology, as a general term for some of the fine red Ancient Roman pottery with glossy surface slips made in specific areas of the Roman Empire; and more recently, as a description of a contemporary studio pottery technique supposedly inspired by ancient pottery. Usually roughly translated as 'sealed earth', the meaning of 'terra sigillata' is 'clay bearing little images' (Latin sigilla), not 'clay with a sealed (impervious) surface'. The archaeological term is applied, however, to plain-surfaced pots as well as those decorated with figures in relief.

Edited January 5, 2023 by PeterH