What is the Secret Behind the Magic Performance of Craft Crank Sculpture Body

Project Name

Craft Crank, Industrial Crank

Project Codenumber

UnAssigned

Notes

Potters often hear stories that a certain clay body is somehow magic. Vitreous yet never warps or plastic yet never cracks. But a closer look reveals the real story, the real compromises that were not conveyed by the original claim. This is an example.

Craft Crank (and Industrial Crank) are two sculpture bodies from PotClays in the UK. Among many these have a legendary reputation for plasticity and drying performance and firing hard and strong despite being heavily grogged. Why? How? The real story can only be told by appraising and comparing them in a lab setting. That is what I did here. What is the difference between the two? Why do they perform as they do?

The secret of these bodies is very interesting:
1) A super plastic, very smooth base clay body. It can host high percentages of grog while still maintaining good workability.
2) A body base that matures well below cone 6, yet when over fired, does not bloat. The coarse grog adds lots of porosity, but none of the total porosity is coming from the body.
3) Very high grog percentage. 40%+. Most other grog bodies have half of that or less.
4) The grog has very few fine particles and the body has a very low coarse particle count. Two extremes put together, no intermediate sizes.

How could we make Craft Crank body in North America? Lots of fine ball clay (to get enough plasticity). Enough feldspar to mature it well at cone 10. Then add 40% coarse grog.

URLs

Craft Crank web page

Craft Crank - From PotClays, UK

Notes

*Stiffness was about right to slightly stiff.
40% grog with 50 mesh predominating.

This is a heavily grogged body. Yet it is sticky, difficult to roll test bars. It is very plastic despite the high grog content. Dry strength is low and dry surface is soft and easy to abrade, this is not unexpected for a body having 40% grog.

The drying shrinage is higher than expected given the amount of grog and the low water content. The base clay is obviously highly plastic and very fine grained. The fired shrinkage is dropping as temperature increases, the opposite of expected. That means the base clay is being fired past its point of vitrification above cone 7.

This is thus a fine grained, sticky, highly plastic, medium fire body which has been stuffed with lots of grog and used at high fire. The result is great workability, good fired strength but fired warping.

Pictures

L3869 Crank Craft, Industrial drying test

This DFAC test creates stresses by unevenly drying the disk. Both of,these bodies have performed very well.

Crank Craft, Industrial Soluble Salts test

Craft on left. Both are displaying significant solubles, likely from a high percentage of ball clay.

Craft, Industrial Crank fired bars

Left: Craft. Right: Industrial.
Top bar is the base (without the grog) fired at cone 7.
Cone 10R bar at top, 10, 9, 8, 7, 6 below.

Crank Industrial - From England

Notes

*Not as sticky as Craft Crank L3868. It is stiff but was not a problem to wedge (water content is lower). Very plastic, capable of pulling very thin slabs.

Dry strength is low but a little better than Craft Crank. Surface hardness of the dry is also very low.

This has about 45% grog and that grog is coarser than the Craft Crank (predominating in 35 mesh). Water content, drying shrinage and fired shrinkage are all thus lower. The porosity is also lower, likely because the base clay is even more vitreous.

Pictures

L3869 Crank Craft, Industrial drying test

This DFAC test creates stresses by unevenly drying the disk. Both of,these bodies have performed very well.

Crank Craft, Industrial Soluble Salts test

Craft on left. Both are displaying significant solubles, likely from a high percentage of ball clay.

Crank Clay Industrial - Closeup of pugged

Craft, Industrial Crank fired bars

Left: Craft. Right: Industrial.
Top bar is the base (without the grog) fired at cone 7.
Cone 10R bar at top, 10, 9, 8, 7, 6 below.

Craft Crank - Base

Notes

*To really understand this body I screened out all the grog from a sample and tested just the base clay.

As expected it is extremely fine, plastic and slick. It has already reached zero porosity by cone 5 and fired shrinkage already headed downward. That means optimal maturity is below cone 5, maybe cone 3 or 4.

How does this compare to the Plainsman 78G? More vitreous and higher dry shrinkage. Fired appearance is very similar to 78G base.

Pictures

Crank Clay base clay- grog removed

Grog has been screened out. These mature well below cone 6. Yet with grog added they are being fired at cone 10. They are highly plastic yet the drying shrinkage is quite low, this is perhaps a secret to why the bodies have such good workability.

Industrial Crank Base

Notes

*This industrial base is less vitreous than the Craft base. Lower fired shrinkage, less over fired.
And it has higher drying shrinkage even though the water content is much lower.

The industrial Crank body thus has more grog, bigger grog in a more plastic, less vitreous base than the Craft Crank.

Pictures

Crank Industrial Grog

That is how much grog is in that piece of clay. This is a very highly grogged body.

Crank Clay base clay- grog removed

Grog has been screened out. These mature well below cone 6. Yet with grog added they are being fired at cone 10. They are highly plastic yet the drying shrinkage is quite low, this is perhaps a secret to why the bodies have such good workability.

Craft Crank Clone 2

Total:100.00 (R)

Notes

*This is an attempt to create a white burning Crank-like body using North American materials (a smooth vitreous base with 40% aggregate).

Although glazes would normally fit this mix of ball clay and nepheline, using 40% grog alone did affect glaze fit (and produced a texture that was just too coarse). This silica sand:grog mix has resolved the fit and produced a much better working texture).

Test bars dried quickly, in one day, so the grog is venting water very well.

Producing a vitreous body by cone 10. Even at cone 6 the porosity is very low considering the amount of grog present.

G2926B white glaze is now fitting at cone 6 (no crazing after weeks).

Pictures

L3868C Craft Crank Clone 2 fired bars

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