PeterH
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Posts posted by PeterH
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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. -
... or maybe you were remembering this. Which I suspect wouldn't
generalisescale to a 30 gallon qvevra.
https://tinyurl.com/36ap7kh8
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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).
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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
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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-approachA slightly odd format as the text is in a pdf file, and the diagrams have to be downloaded separately.
.jpg)
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48 minutes ago, starrs_ridge said:
I had not thought of that -- do you mean for the incoming 240v current? Or at some other point in the circuitry?
From https://tinyurl.com/jtcjby88
We also purchased this $34 amperage detector from Bartlett which is a tiny circuit board you connect to the kiln controller and has two white leads connecting to a ferrite ring.
You thread one 220 volt power lines from each relay to the heating elements through the ferrite ring, which measures the power being consumed. This allows the controller to compare your current power draw to the original power draw when you installed the elements or in our case when we installed the controller.
The controller tests the elements for about 60 seconds prior to each firing. A decline in amperage used is charted to determine your element aging and any sudden decline indicates the failure of an element.
I think it also does other helpful things, like reporting the power consumed (and its cost). Don't know if it can detect things like stuck relays in real time.
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Are you going to fit a current sensor?
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1 hour ago, neilestrick said:
This also assumes that the dry clay is the same cost per pound as the moist clay. When I worked for A.R.T., we charged more for dry mix because mixing and bagging dry clay was a much slower process for us than making moist clay.
Important practical point well worth emphasising, but I did say per lb.
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On 8/20/2022 at 6:37 PM, NancyE said:
And thank you Min, 25-50% sounded excessive and did not jive with what I did with my reclaim.
Just for the record (as there are obviously bigger issues involved).
Assuming Min is right and wet clay contains 20-23% water -- and if dry clay contains none -- I think you should get 25-30% more clay per lb.
20/(100-20) = 0.25 = 25%
23/(100-23) = 0.2987 ~30%
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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 mlNow 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?
- Rae Reich, Bill Kielb and Min
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Haven't been able to find anything more about this:
http://www.potters.org/subject98825.htm
Frank Colson on sun 4 feb 07
Some many years ago, I published an article called: Total Wet Firing. I
would take a freshly thrown pot off the wheel head and fire it immediately
in a preheated raku type kiln Did a workshop for a group of local art
teachers, so we were able to "experiment" this type of firning with more
than 100 pots so we were able to get all te wrinkels worked out. You might
call
the approach a one-upmanship raku technique. Allthough I still have one pot
on my shelves, it was great fun but not very peranent. The entire firning
proces took about 5 minuets!
If anywone would like to see how to do "Total Wet Firing", I'll put it on my
website as a free download.
Frank Colson
www.R2D2u.com -
On 8/16/2022 at 12:45 PM, freddyfrink said:
Hello, I am doing some research on these ceramic plates that I bought.
Customer service informed me that the plates were made of: "The stoneware is made of clay, feldspar, quartz and the coating is composed of feldspar, calcspar, kaoline, barium carbonate and pigment."
Has anybody any idea why there would be barium carbonate in a black glaze? ... on second thought maybe it's to give a velum matt finish.
If the barium isn't significantly effecting the colour, and the glaze is already matt, would this minimise the chances of a vinegar test showing anything (even if barium was being leached)?
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2 hours ago, Babs said:
No scientist, and just rwmember reading If folk who put pots i kiln straight off the wheel, sealed kiln and fired.
Wonder what the firing schedule was, prob pre controllers.
Will try to find that source.
Off topic sorry Min.
I vaguely remember Ian Gregory (of fibre kiln fame) making a similar comment during a demonstration -- about being able to fire a kiln-full of freshly thrown pots without trouble. He then immediately said that with somewhat dryer pots this didn't work.
The impression I got that there was a brief sweet-spot where this was possible (and then only for a kiln-full of such pots), not that the state of dampness wasn't important.
I've no coherent theory to explain/understand this claim. Although its possible that some sort of wicking process draws the water to the surface as the pot dries, and the large number of wet pots in the kiln limits the temperature ramp while there is still surface water to evaporate. Surface drying before firing could prevent such wicking of water to the surface during firing.
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This earlier post by @Jeff Longtin may be of interest
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Perhaps of mild interest, despite cryptic translation.
https://patents.google.com/patent/CN1055278C/en
Method for making colour glaze natural leaf figure ceramic product
Abstract
The present invention discloses a method for making color glaze natural leaf pattern ceramics. Natural leaves or plant leaves are stuck to a blank coated with base glaze after particularly treated, and a layer of surface glaze is coated on the blank. Then, the blank is put in a kiln to be burned to obtain a ceramic finished product. The present invention uses ceramics as a carrier, and natural leaf patterns are burned on the ceramics. The leaf patterns are clear, natural and vivid and have beauty sense and very high appreciation and collection value. The present invention has no particular requirements for conditions of the base glaze, the blank, combusting atmospheres, etc. and has the advantages of simple preparation process and easy operation.
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2 hours ago, jayjay said:
Does anyone have a copy of this thesis. It seems the link is dead.
Hang on ... it's still out there. Usable link below
A google for TRANSFORMATION_OF_SIX_LEAVES_GLAZE_TO_CONTEMPORARY_CERAMIC_BASES_ON_THE_TAOISM_CHARACTERISTICS filetype:pdf
gets me to an Resource not found error page at
http://www.sure.su.ac.th/xmlui/bitstream/handle/123456789/20993/DR_FENG_Shanxin.pdf?sequence=1&isAllowed=yClicking on Go to SURE home get me to the sites home page at http://www.sure.su.ac.th/xmlui/
From there a search for TRANSFORMATION OF SIX LEAVES GLAZE gets me to the pdf at
http://www.sure.su.ac.th/xmlui/bitstream/handle/123456789/24111/DR_FENG_Shanxin.pdf?sequence=-1&isAllowed=yI give the search details as it's possible that you may find other items on the site of interest.
PS ... and an alternative URL, possibly to the same instance of the file :
http://ithesis-ir.su.ac.th/dspace/bitstream/123456789/2977/1/60155902.pdf
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1 hour ago, oldlady said:
just a question from a cone 6 user. what is a kiln setter? i skimmed the article and see the pictures, is it also known as wadding? the stuff used in cone 10 firings? or maybe it is used as a plate or tile rack? i see that it is not the usual mistake of calling a kiln SITTER a kiln SETTER. what are they for?
Setter is a name for types of kiln furniture which offer some protection against slumping/distortion.
https://www.earthwaterfire.com/products/setters/
Ceramic Setters are special items of kiln furniture which, in the ceramic tableware industry, are shaped (generally on their upper surface) to conform with the undersurface of plates and dishes and which serve to support ware and maintain its shape in the kiln during
Setters are especially important when firing bone china.
https://ceramicartsnetwork.org/pottery-making-illustrated/pottery-making-illustrated-article/Tempting-Transparency#
In cases where the bisque firing is much higher than the glaze firing, as is normally when working with the case with bone china (between 2246–2300°F (1230–1260°C)), it’s essential that the clay ware receives the maximum support during the firing process, particularly as most of the movement occurs when the kiln reaches its highest temperature. Due to the high distortion factor encountered with bone china, if the shape is to be controlled, any open form requires a setter of some kind.If the form is circular at the top, it can be placed upside-down on a refractory clay ring, or setter, with an alumina wash to prevent sticking in the firing.
If the form is irregular at the top edge an individual setter needs to be made. In simple terms, this is an item that resembles a lid and is cast separately (the preferred making method for bone china). Its function is to contain the top edge, or rim, of a form during firing (figure 10) and is discarded afterward. If the piece has been successfully fired with no distortion, the ware can be glazed and fired at a lower temperature of 1832–1976°F (1000–1080°C), if necessary.
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6 hours ago, Kelly in AK said:
Vince Pitelka has an orifice chart on his website. Ward burner is a font of wisdom in this department.
http://www.vincepitelka.com/handoutsinformation
/https://www.vincepitelka.com/wp-content/uploads/2019/02/Low-Pressure-Gas-Orifice-Chart.pdf
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1 hour ago, Pres said:
Suz, I would look up the orton cone chart, and compare the temps for the self supporting and small cones. I believe you will find the small cones fire hotter than the self supporting.
5 minutes ago, suz davis said:i did a bit of research and you are correct. the firing temps of self supporting and small cones are different.
Can somebody point me to an Orton chart which shows the temps for small and self-supporting cones when fired at the same rate?
PS
The first chart I found gives the (different) cone temps for small cones at a significantly higher firing rate for the small cones.
... this page, while referencing the same Orton charts, implies that small and self-supporting cones operate at the same temps (when heated at the same rate).
https://nmclay.com/informational-pages/orton-cone-chart-in-centegrade
... but gives no Orton reference for this -
Do you still have access to the clay and wheel at college?
If so might be worth trying home clay wedged at home on the college wheel, and vice versa. It might at least remove some variables.
... and carefully consider any differences in body/arms/wheel/chair relationships in the two situations.
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Skutt GlazeTech kiln issues. Relay still clicks but output voltage not as expected?
in Equipment Use and Repair
Posted · Edited by PeterH
That confuses me too. IIRC you measured 243V in and 16.5v from one arm of the output.
As the transformer is nominally 230v to (12+12)v surely you should have been reading (240/230)*12v= 12.5v. It's either a massive reading error, or the transformer isn't working as advertised. Barring shorts inside the transformer, or substantial "stray" currents through one of its coils this seems wierd.
If the joints are easily breakable/re-makable it would be interesting to measure the transformer outputs open-circuit (and recheck it's inputs).
... and confirm the voltage across the relays input is now zero.