Large vessels

[NancyE]

Hello guys, I'm back trying to make the Qvevri.  Does anyone know how I can determine how thick I need to make a bisque fired terra cotta wall to hold the pressure of the liquid contents inside?  Of course I'm not making the huge qvevri that they made in Georgia and had to be buried, still might try for a 30 gallon vessel.  This is assuming my seams are good, walls consistent, my recycled clay was clean etc.  And also, I'll probably end up pit firing because of the whole size of the pot kiln situation.  The shape will be a variation of the classic qvevri, round with high and wide hips, but probably not so exaggerated.  And it will be sealed with beeswax.   As always I appreciate your wisdom, input and encouragement.

Nancy (now dba Barefoot Arts)

[Rae Reich]

Hi, Nancy! Glad you’re back to your project. Technically, I think it’s not the thickness of the walls but the soundness of them that keeps vessels whole under the pressure of the contents. You want to have a consistent thickness and no cracks for good drying and firing. A five foot tall jar might need up to 2” thickness, a two foot jar maybe less than 1”, but that is for strength during construction and firing, not strength against internal pressure, which comes from no cracks or fissures. (Burial was both the preventative and the cure for ancient cracked vessels.)

There are YouTube videos of coiling large jars. Generally, the coils are about 1”-1 1/2” in diameter - human hand scale. 
 

[neilestrick]

I wold think that even at 30 gallons they would not need to be especially thick. Clay is strong stuff. Just picturing a 30 gallon trash can, 1/2" walls should be able to hold it. It would be difficult to build that thin, though, so I would just do what you can and not worry about it.

[Pres]

Nancy, are you looking to make yours with the pointed/rounded base or with a flat base? I believe that part of the process is determined by the base. Thickness of the larger pots seems to be 2-4" in coils, and then the scraping will bring the walls a little thinner. flat bottoms have more stress on the bottom for weight of the walls whereas the rounded or pointed bottoms are supported with the side walls with different stress points. Interesting project. . . wish we could help more. I would think your best efforts would be to work smaller and ramp up when  you get more assured/confident of your process.

 

best,

Pres

[PeterH]

I've no personal experience on making large pots, but I'd like to give a few estimates of wall thickness. I'll use metric units (30 US gallons is about 114 litres).  

1) The long history on amphora for both storage and transport.
Digital modeling of function and performance of transport amphorae
https://ceramics.onlinelibrary.wiley.com/doi/full/10.1002/ces2.10056
... which describes relatively small vessels from 26.8 to 44.5 litres, states
The indicated weights of the empty vessels correspond to a uniform wall thickness of 10 mm and an assumed density of the ceramics of 1.7 g/cm³.
I find the thinness of the walls a little surprising considering the stresses and strains that would occur when they were used for transporting goods. Where they would encounter a fair degree of man-handling and other stressful environments.

2) Qvevri shapes and dimensions.
Wine and Food Travel: Qvevri Wine Making Tradition of Georgia
https://www.indianwineacademy.com/item_6_622.aspx
... which links to a larger version of this image showing the relationships between the various dimensions of a family of Qvevri.

This states that the wall thickness is between 2 and 5 cm. The upper figure being quite low considering the size of some vessels.

Although Quevri leave largely static lives (especially the well-supported buried ones)  the wall thickness has to support man-handling during construction and while moving to its final location. The neck area in particular needs to be suitable for the use of ropes.

It also gives an approximation to the volume of a Qvevra, which is the same as that of an ellipsoid of the same width and height.

 

 

3) Modern Qvevri for sale

From http://www.domainegeorgia.com/order.html

The 120 litre one is closest size to 30 US gallons, and I'll try to estimate its wall-thickness from the information available, modelling it as an ellipsoid (in units of ml and grams).

v  = volume of an ellipsoid with same width and height = 4/3*pi*(50/2)^2*55
c = internal volume with wall of thickness t = 4/3*pi*(50/2-t)^2*(55-t)
w = volume of clay walls = 60*1000/(density of ceramic) = 60*1000/1.7 ml

Now  w = v - c giving
60*1000/1.7  = 4/3*pi*(50/2)^2*55 - 4/3*pi*(50/2-t)^2*(55-t)
v = 4/3*pi*(50/2)^2*55
c = 4/3*pi*(50/2-t)^2*(55-t)

Sticking these into a symbolic maths package such as  https://www.dcode.fr/equation-solver gives

So the estimated capacity is fairly close to the advertised 120 litres, and the estimated wall thickness 2.72cm (1.07 inches) for a real-world  Qvevra of ~120 litres. Again surprisingly thin considering the manhandling it will get, although  (unlike amphora) it is only moved while empty (and lighter).

4) Comments on building and shape.

Fairly obvious comment, they do seem to "work to coil" heavily, presumably to minimise weak-spots due to non-uniformity. As can be seen in  https://www.redfedoradiary.com/qvevri-georgian-wine/


The conical bottom to the interior seems to be functional (grape seeds fall into it and are covered in lees, keeping bitter compounds from entering the wine). Prompted by Pres's query I realised all the pictures of making Qvevri I can remember have the potter walking round a fixed pot. Considering the size of large Qvevra putting "pointy" ones on any sort of turntable might be difficult, balancing them while turning it even more so. They usually seemed to be propped up on the ground like so:

Does anybody know how/if smallish pointy Qvevri can-be/are built on a wheel?

 

 

 

 

[Bill Kielb]

Just a thing or two to think about: 30 gallons of water weighs about 250 pounds. The pressure on the cylinder walls will be dependent on the height of the liquid. (0.433 - about 1/2 psi for each foot). And of course the container itself will be the weight of the container plus the weight of the water so picking this up with two hands will place a lot of force where your hands are placed on the cylinder. Assuming 50-100 pounds of clay, this container full will likely be quite a challenge to pick up and move around.

[Pres]

I have never seen larger pointed vessels thrown as all have seemed to be coil constructed with a lot of paddling and compression of layered coils. 

On another thought, I read somewhere that the shape of the Queri/amphora was to allow tilting of the container to pour into other containers for use. 

 

best,

Pres

[Rae Reich]

Thanks, @PeterH, for the cool research!

I’ve been speculating about the supports used as the quevri  are formed in the article on Georgian production. The irregular, yet effective, legs look a bit like stone or wood.
Anyone know for sure?

[Callie Beller Diesel]

Not for sure, but they look a little like broken terra cotta roof tiles. Could be that they’re using some kind of waster they just had around. I imagine that you could cut some from wood if that’s the material you have to hand.

 

[PeterH]

Stumbled across this while researching my last post. In case anybody is interested ...

Throwing Punic Amphorae: An Archaeological and Experimental Approach to the use of the Potter's Wheel in southern Iberia during the Iron Age
https://exarc.net/issue-2021-2/ea/throwing-punic-amphorae-archaeological-and-experimental-approach

A slightly odd format as the text is in a pdf file, and the diagrams have to be downloaded separately.

[Pres]

@PeterHI have seen something like this before, but it was a wheel driven by a man below the ground while thrower was above ground. Don't remember where, but in one of my many books

 

best,

Pres.

[PeterH]

Wow, it only takes 10 mins.
Throwing/ Making a Onggi Pottery(Part 1 of Part 2)Korean Potter

[PeterH]

15 hours ago, Callie Beller Diesel said:

Not for sure, but they look a little like broken terra cotta roof tiles. Could be that they’re using some kind of waster they just had around. I imagine that you could cut some from wood if that’s the material you have to hand.

Good spot.

Several nice examples in https://wander-lush.org/clay-qvevri-wine-shrosha/ including

... and support in the kiln with bricks & small stones

[Babs]

Why not constructed/thrown upside down?????

Or literally from wheel , who was that guy , who threw "downwards" at a certain stage of the process

[PeterH]

https://youtu.be/2xOGWsFJ4DA

14 hours ago, Babs said:

Why not constructed/thrown upside down?????

Or literally from wheel , who was that guy , who threw "downwards" at a certain stage of the process

I'm interested if you remember any more details . The closest thing I've found yet is this:

Video at  https://youtu.be/2xOGWsFJ4DA

I wonder if you can do the same thing with coil-building (which may be necessary/helpful for larger pots).

 

[PeterH]

... or maybe you were remembering this. Which I suspect wouldn't generalise scale to a 30 gallon qvevra.

https://tinyurl.com/36ap7kh8

Edited August 24, 2022 by PeterH

[Babs]

9 hours ago, PeterH said:

... or maybe you were remembering this. Which I suspect wouldn't generalise scale to a 30 gallon qvevra.

https://tinyurl.com/36ap7kh8

Yes that is the one.

I still wonder why the vessel isn't constructed upside down, but I guess it evolved and was designed by greater folk.

Today with the tools to lift and flip vessels to aid assembly of big pots, different methods become possible.

Wouldn't want to be the one who accidently kicked one of those supports out from under...

[PeterH]

An old clayart post giving a selection of tips for throwing and joining  large-ish pots in sections.
throwing tall pots http://www.potters.org/subject09085.htm
Including the use of plastic-wrap: to eliminate a great deal of the time required for the lower portions of sectional pots to set-up before attaching and working the upper section(s).
... no idea if this would have any relevance to coiling thick/heavy  pots as well.

[Babs]

1 hour ago, PeterH said:

An old clayart post giving a selection of tips for throwing and joining  large-ish pots in sections.
throwing tall pots http://www.potters.org/subject09085.htm
Including the use of plastic-wrap: to eliminate a great deal of the time required for the lower portions of sectional pots to set-up before attaching and working the upper section(s).
... no idea if this would have any relevance to coiling thick/heavy  pots as well.

So many tunnels in this rAbbit hole, and now we can use heatguns to firm up the sections or light a fire inside like @Pres

[NancyE]

You guys are great, there is so much more information available than when I started on this even 2 years ago.  Hi Rae:)  Thank you Neilstrick, Babs, Bill and Callie .  Peter H, some very interesting new information on the amphora.  I am re-designing and expanding my studio, because there is barely room for me and even one "large" Qvevri.  The Georgians make it look so easy.  I made one I think would hold about 30 gal, but it took so long the bottom dried too much and began cracking, so I will smash it to reclaim to the clay.  I keep putting if off just because.  I have been experimenting with almost all these above methods and different designs.  My last two pots required so much pressure to shape the mass of clay, working stiff to keep it from collapsing, I had trouble keeping them  from sliding off the wheel head.  What I was doing is a kind of coiling with trimming and shaving with ribs.  I wish I had longer arms.  Considering how many times I've been clocked by one of my big pots, I don't see how throwing upside down they can keep the pot stuck on the bat!  Pres, of course the pointy bottoms remove stress on the walls, it just never occurred to me since the pointy part seems so vulnerable.  Most "prehistoric" pottery has round bottoms, I always assumed it was due to the lack of tables, maybe it was for smarter reasons.  Thanks again, Nancy E

[Rae Reich]

Hi, Nancy 
I’ve seen plastic wrap used to slow drying on the bottom.  Might have to re-wet the clay under it for good sticking. 
 

Your pots look great!!

Edited August 30, 2022 by Rae Reich

[Babs]

Then there arevthe guys who have a wooden frame work, collapsable , and totally covered with ropes. After the pot is built around the above the frame is collapsed, ropes unwound and there you go. French or Spanish I think.

http://www.poterie-provencale.com/savoirfaire_gb.swf

[Babs]

https://www.instagram.com/tv/COz6edTFy6Y/?igshid=YmMyMTA2M2Y=

Check out his other posts. Big pots.

[Kelly in AK]

When it comes to large pots I’ve always been smitten by the Cocuchas, from Cocucho in Michoacan. The second link has some process photos. It’s a man making those pieces, but I’m under the impression it’s traditionally women who make them. 

https://mansioniturbe.blogspot.com/2017/10/cocuchas-distinctively-michoacan.html

https://www.mexconnect.com/articles/3633-david-santos-alonso-ceramic-art-in-the-mexico-town-of-cocucho/

[NancyE]

I used plastic wrap, but I also took 3 months and tried to reshape the pot by adding and removing from the body.  I just took too long.   I was unable to open the link to the guy with the wooden frames, but I remember seeing them before, and considered what they were doing.  I think I just need to get my studio in order, and work better.  Whenever I watch the videos of the beautiful large pots being made around the world I'm a quite ashamed of how much I think of mine, and how much I struggle, but I've got to stop that and keep working.  The women from Santos Alonso really touched me with their gorgeous pots, and because I worked with Mexican immigrants for so long.  Perhaps I'll take a trip someday.  NancyE