Clay thickness, in relation to physical strength

[glazenerd]

2 hours ago, Magnolia Mud Research said:

 

"Undissolved quartz was present in Glaze B.  The quartz caused an increase in the thermal expansion of the glaze.  When the glaze was fired to a higher temperature and all of the quartz was dissolved, the thermal expansion of the glaze decreased.  Therefore, it is not always true to say that increasing the amount of silica in a glaze will lower its thermal expansion.  This is only true if there is no undissolved quatz (sic) present in the glaze" [Benson, Jennifer L. "Effects of Glaze Variables on the Mechanical Strength of Whitewares." (2015). pg. 67] 

LT

 

LT

Lawrence addresses this issue in his book: Material Science for the Potter.

free silica reacts much differently than amphorous glass. Most clays have up to 1/3 free silica, some higher, some lower. The flaw in glaze calculators: no variables for this issue. Sodium based glass (soda glass) is softer to begin with. Lots of variables on the original question: clay wise- mullite development will give the most strength.

T

[Bill Kielb]

Never was too fond of the calculated COE this seems to support that view a bit and also that glazes melt as a result of their composition so certain limits developed for certain cones a bit iffy as well. Interesting stuff!

[glazenerd]

Perhaps a real world application.

Low fire bodies (06-04) commonly have tested COE values from 6.50 up to 9.00. Laguna EM series is a good example: EM 337 , low fire sculpture comes in at 9.50. Yes, grog plays a role: but all the silica in a low fire is no where close to being melted. They make talc additions in low fire to handle thermal expansion: and now you know why. Silica lowers COE if it is fully incorporated into the amphorous structure: if not, it can raise COE..a lot!

Tom

[Benzine]

@neilestrick  the second school district I taught at, got materials, for our Stained Glass class, at that shop.

I didn't realize you spent time here in the Hawkeye State.

[neilestrick]

Just now, Benzine said:

@neilestrick  the second school district I taught at, got materials, for our Stained Glass class, at that shop.

I didn't realize you spent time here in the Hawkeye State.

Small world!

What I was getting at with the glass info, is whether or not thickness really affects strength all that much. Once a weakness is established, will it just run through the body like scored glass? Is it more difficult to establish that weakness in a thicker piece? In Pinnell's test, which was done by pressing on bars spanning two supports, would a thicker piece actually be stronger than a thin piece, or do they all crack through at the same degree of pressure regardless of thickness? Are thin pieces more flexible, like wood?

[glazenerd]

2 hours ago, neilestrick said:

Small world!

What I was getting at with the glass info, is whether or not thickness really affects strength all that much. Once a weakness is established, will it just run through the body like scored glass? Is it more difficult to establish that weakness in a thicker piece? In Pinnell's test, which was done by pressing on bars spanning two supports, would a thicker piece actually be stronger than a thin piece, or do they all crack through at the same degree of pressure regardless of thickness? Are thin pieces more flexible, like wood?

I have a 2500x scope upstairs, but no USB camera on it. Amazing actually what you cannot see with the human eye. I have seen grazing lines on the interior of porcelain.  Common to see iron saturation " spots" in stoneware bodies: which means the COE values in those areas are lower.  Two things I have observed: the thicker the wall, the porosity is higher in the center than the outside. Our hands, slab rollers, etc. just do not have the compressive strength. The glass content changes from the outer to the interior. 

I read a study done years ago by an engineer in Brazil on thermodynamics and wall thickness. The most notable citation being: in a 1/2" thick wall: it can take an additional 30 minutes for temp to equalize to the kiln atmosphere. Potters prove that when they fire: adding a hold time or slowing down the ramp on the upper end to allow spars to off gas. The further the heat has to penetrate, the longer it takes for temps to equalize. 

T

[glazenerd]

I see these defects all the time on thicker pieces. Slab roller just does not have the compressive strength to compact them. You can also tell by the discoloration at the center cavity that heat work was not complete.

[Bill Kielb]

Cool pics!

Ya mean ceramic products are not necessarily perfectly homogeneous? I am shocked!

[Tyler Miller]

This book might be useful in this discussion:

Munz, D., Fett, T.: Ceramics, Mechanical Properties, Failure Behaviour, Materials Selection, Springer, 1999.

 

From the dust jacket:

Ceramic materials are widely used as components in a great variety of applications. They are attractive due to their good high temperature strength, high wear resistance, good corrosion restistance and other special physical properties. Their major drawback is their brittleness and the large scatter of mechanical properties. This book describes failure phenomena in ceramic materials under mechanical loading, methods for determining the material properties, and the principles that one should apply when selecting a material. The fracture-mechanical and statistical principles and their use in describing the scatter of strength and lifetime are also covered. Special chapters are devoted to creep behaviour, multiaxial failure criteria and thermal shock behaviour.

[glazenerd]

Bill: 

People often ask what I would spend lottery winnings on? I could easily drop $500,000 on testing equipment. Only draw back: I need to actually buy a ticket. I see that midline a lot on thicker pieces. One of the reasons I use potassium in test pieces: I can track it by discoloration. Unspent potassium always leaves behind a vapor trail.

Tyler there is a book that might actually be useful to me.

[Magnolia Mud Research]

Tyler, thanks for the book reference

Here are two links that will add some insights, and also more questions, to the discussion of both strength of pots and other ceramics science. Have fun! 

LT
Captions from the two links:

Materialwissenschaft I (327-0301-00)
Goal:
To give a profound knowledge of the atomic and the macroscopic concepts in materials science.
Contents (for the ceramic part of the lecture)
historic overview on ceramic materials and their processing
crystal structures of ceramic materials
Al2O3, ZrO2, Si3N4 and SiC as most important structural ceramic materials
and their structure, chemical bonding, and physical properties, phases and phase diagrams
glass and glass ceramics
mechanical properties (strength, slow crack growth) and their statistical description
Recommended Readings:
For ceramics part in Materialwissenschaft I and II see
D. Munz, T. Fett, Ceramics, Springer, ISBN nr: 3-540-65376-7 and some of:
Physical Ceramics; Y.-M. Chiang, D. Birnie, D. Kingery, Wiley, 1997. Or
Modern Ceramic Engineering; David Richerson, Ed. 2, Dekker, 1992.
M.W. Barsoum; Fundamentals of Ceramics; IoP Publishing Ltd, 2003
Brevier Technical Ceramics

http://www.nonmet.mat.ethz.ch/education/courses/Materialwissenschaft_1 

Materialwissenschaft II (327-0401-00 )
Goal:
The aim of the first part of this lecture is to get insight in the description of mechanical properties of brittle materials and their statistical nature including toughening concepts for ceramics.
Contents (for the ceramic part of the lecture):
Introduction to brittle behaviour of materials (ceramics)
Stress intensity factor
Flaws in ceramics and fracture strength of components
Slow crack growth
Life time prediction of ceramic components under cyclic load and design guidelines for ceramic components: A case study
Proof testing
Creep
Thermal shock
Toughening concepts for ceramics
Recommended Readings:
Fundamentals of Ceramics, M.W. Barsoum, IOP Publishing Ltd, 2003
Mechanical properties of ceramics, John B. Wachtman. New York [etc.] : Wiley ; cop. 1996.. XXII, 448 p. ; 25 cm : ill.. [001660538]
Ceramics, Mechanical Properties, Failure Behaviour, Materials Selection, D. Munz, T. Fett, 2nd edition 2001, Springer
Fracture of Brittle Solids, B. Lawn, 2 nd edition 1993, Cambridge University Press
http://www.nonmet.mat.ethz.ch/education/courses/Materialwissenschaft_2 

 

[Tyler Miller]

Solid bibliography, LT! Looks like there’s a newer edition of the book I recommended in there.  I’ll confess I probably got the original citation from a course biblio like that. 

  A habit I’ve gotten into for research is to assemble a file to cut and paste citations into for later ref.  Journal articles cite books or other articles I can’t read at the time, but feel might be useful, so i copy and paste the citations into a file.  You end up getting a solid bibliography on a topic pretty quick.

Sometimes a mailing list is a help for new stuff, if you can get on a publication review mailing list.  I don’t know of  ceramics one (maybe someone here does), but the Bryn Mawr Classical Review is forwarded to a file folder in my google account that now works as a searchable database for all forthcoming work.

For those looking to buy these books, I recommend something like abebooks.  Academic books are stupid expensive and hard to find, and for studio purposes the most current edition doesn’t matter.  Universities are great for getting around academic journal paywalls and you can usually access them from whole if you have an alumni password.

[Magnolia Mud Research]

3 hours ago, Tyler Miller said:

Special chapters are devoted to creep behaviour

Tyler,

I overlooked the "Special chapters are devoted to creep behaviour" in your earlier post;  I'll have to get the book to just for that.  I have a hunch that the bending of pyrometric cones are just another example of materials creep. 

LT
 

[Bill Kielb]

Much simpler than that,

Cones are made of glaze , gravity takes over when they are soft enough.

A cone six cone,  is 0.6 alumina, 6 .0 silica with fluxes at 0.3:0.7.

cone 4 is roughly a cone 10 glaze. Seger just sought to verify glaze maturity points.

[Tyler Miller]

1 hour ago, Magnolia Mud Research said:

Tyler,

I overlooked the "Special chapters are devoted to creep behaviour" in your earlier post;  I'll have to get the book to just for that.  I have a hunch that the bending of pyrometric cones are just another example of materials creep. 

LT
 

I sortof thought it fell under the category of pyroplastic deformation, but creep might be an interesting way to look at it?  

I don’t know how useful those chapters will be in answering that, honestly.  The reason I recommended it is that it talks about what the author calls “scatter of mechanical properties,” which is essential to understanding the behaviour of ceramic, in addition to the chapters “Fracture mechanics” and “Determination of Strength.”

There are a lot of books that seem to be called “Mechanical properties of Ceramics,” but one with a more general chaper on deformation (as well as fracture):

Pelleg, J.:  Mechanical Properties of Ceramics, Springer, 2014

It’s geared toward an undergrad audience, though?

 

[Magnolia Mud Research]

4 hours ago, Bill Kielb said:

Cones are ... , gravity takes over when they are soft enough

You are very very close to understanding creep deformation.

LT

[Bill Kielb]

I am far better in its application to concrete as its effect generally has catastrophic potential implications. For cones,  Seger ingeniously worked out the maturity point of Something  that represented the glaze on his pots. Pure simplistic genious to which Orton brought to the states and made his mark selling Seger’s ingenious way of figuring out heatwork.

I never want to study creep deformation, had too many in the blue collar neighborhood I grew up in. LOL!