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Member Since 06 Apr 2010
Offline Last Active May 23 2017 01:23 PM

#125869 Spiral Crazing On Base

Posted by JBaymore on 27 April 2017 - 09:28 AM

A glaze either fits the body or it doesn't.  To "fit" the COEs (or COTE .... or CTE) of the clay body and the glaze need to be close enough so that the tensile strength of the glass produced by the glaze formula is not exceeded.  COE is abbreviation for "Coefficient of Thermal Expansion".  Note that this "thermal expansion" in this case is NOT directly related to the large amount of permanent firing shrinkage that a clay object goes thru when fired. 


I also like to add the word "reversible" in that title for clarity of concept: "Coefficient of Reversible Thermal Expansion".


All materials expand a bit when they are heated and contract when cooled.  In the kiln, once a glaze has "frozen" after the melting process is completed.......... it can be considered a "solid" (even though is really is not exactly that).  The clay body is a solid (even though it has some glassy phase within it).  Each of those things will shrink a certain percentage of their linear size as the kiln cools. 


If the clay shrinks LESS than the glaze, and hence you could also say the glaze shrinks MORE than the clay, as the object heads toward "room temperature" it puts the glaze in tension.  It "pulls" on the glaze layer.  If this pulling exceeds the ability of the glaze to resist... it cracks to relieve the tension.  This is crazing.


Crazing is when the glaze can't "resist" the tensile pull.  Where the glaze is thinner in application... the is not as much glaze "pulling" on the rest of the glaze layer.  But where thicker....... it can reach that, "I canna' hold er together Cap'n.  She's breakin' up" point.  And the thick areas show crazing.  This tells you that the glaze does NOT fit the clay body... and delayed crazing will happen on this object.


If the fit between the glaze and the clay is CLOSE... but not close enough... you get what is called "delayed crazing".  This is displayed when the pieces come out of the kiln looking just fine without any crazing, but in use, the crazing develops.  This is because the glaze is sitting there in tension close to its breaking point.  Heating and cooling in daily use or just from atmospheric temperature changes finally causes the glaze to say "enough!", and let go.  Cooking and food use is a great way to stress the pieces to have them let go.


I've seen this "spiral" business on the feet of earthenware before.  And yes, it is showing the radial stresses in the forming of the wet clay piece... as the fired clay "remembers" some of the alignment of the clay particles that happened during forming.





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#125776 Qotw: The Power Grid Has Gone Down In Your Area A...

Posted by JBaymore on 25 April 2017 - 12:11 PM

I've been mostly "off-grid" for most of my ceramic life.  My first wood firing and wood kiln was summer 1969.  I've used gas fired kilns with venturi type burners (no power needed) and mostly wood fired work.  I bisque in gas... finish fire in wood. My noborigama here at my studio is 40 years old now.  The gas kiln is one year older.


I do use electric kilns for overglaze enamels though.  But if necessary... I'd build a simple updraft wood-fired muffle kiln to fire the enamels in.


I have 6 kickwheels in the studio (selling off 4 shortly.....don't do workshops here anymore).  One of them a wooden Japanese one (keeping that of course).   So throwing would be OK.


Have a river on my property... so some form of water power for some pottery aspects would come into play.  Land for gardening.  House heats with wood.  Have some solar.  Have some woodlot.  We'd do OK.


Are we going all dystopian here? ;)





#125769 Recommend Me: A Glaze Material Book For Foundation Principles.

Posted by JBaymore on 25 April 2017 - 10:13 AM



Just as a "gauge" for you in your thinking about approaching this study........... 


In my typical first level, required, BFA degree 'Glaze Chemistry' course I teach, which lasts for 15 weeks a semester....... two formal 3 hour classes a week........ one being a "class room" session and one session solely a lab work session........ plus their homework time outside of class time............


They mix up and fire a LOT of tests.  All are applied onto 3-d forms... not to flat tiles.  Flat test tiles are great... if you are making tiles... or maybe plates.


Unfortunately... nothing takes the place of lots of hands-on testing work.  "Book-learning" is very important.... but to really understand what the books are saying.... you need objects to relate that information into context.


Also........ the very first testing assignment is simply to find a glaze recipe in a book, online, from a friend, etc. that you really like.  I then use that recipe they have chosen to address the concepts of "recipe" and "formula", to broadly look at raw materials and their compositions, to introduce oxide based versus materials based approaches, to introduce glaze lab working procedures, to hit stuff like the impacts of significant figures and error factors, and so on.  They mix that glaze up and apply it in different ways on 3-d forms.  THEN they will live with that recipe for a while....... working on structured variation to it to make specific changes.  And analyze those changes at both the materials level and the oxide level.


This is just one part of the materials course.  Yeah... they do a lot of testing.  And learn to use Insight software.


They can optionally also take a Level II glaze chem course.  All are required to pass the level 1 class to graduate.





#125676 Recommend Me: A Glaze Material Book For Foundation Principles.

Posted by JBaymore on 22 April 2017 - 06:18 PM

You are not going to find your list all in one place (unfortunately).  Digitalfire is a great resource particularly if you join.  The good side is that Digitalfire is constantly updated..... books go "stale" the minute they are printed.  As the field's knowledge base moves forward.... the stuff in books does not necessarily keep up.


The book you listed is a good one.  Also look at "Out of the Earth Into the Fire" -??????, "Ceramic Science for the Potter" -Lawrence, "Ceramic Glazes" - Parmelee.





#125634 Pottery Knowledge Quiz Of The Week (Pkqw): Week 4

Posted by JBaymore on 21 April 2017 - 01:18 PM

Pottery terminology varies by locale.  "Onglaze color" in the UK is sometimes what we in the USA call "washes".





#125627 Finally Got Some Clay And Glaze To Match

Posted by JBaymore on 21 April 2017 - 10:00 AM

no wonder the sung bodies have 1 % to 2% iron in them  







#125591 Quick Question: Sea Shells - Cone 6 - Which Type?

Posted by JBaymore on 20 April 2017 - 09:26 AM

Being a woodfirer who uses shells a lot as separator wadding (and decoration).....................


The impacts of the shells for attractive scarring/marking come from two main factors. 


One is that the calcium hydroxide mentioned above turns into calcium oxide at elevated temperatures in the kiln.  Calcium oxide is a powerful flux on silica at high temperatures.  All clay bodies contain silica.  So where the shell material is, it provides a large amount of CaO to react with the SiO2 on the surface of the clay.  This causes some of the silica there to melt.  BUT..... calcium carbonate itself is pretty refractory.  So the vast excess of what is needed to flux the small amount of silica on the surface of the clay acts as a non-melting "separator" wadding.  When stacked on glazed areas, the effect is similar... but the excess of calcium oxide also "matts out" the glaze around the shell where there is "too much" ... and the glaze gets a textural marking that can be nice also.


Secondarily, seashells come from........... wait for it.... the SEA.   So when the animals die of natural causes... the shells knock around the ocean for a while before getting picked up on the beach.  The shell surface "deteriorates" a bit since the shell is no longer "living".  The shell get a little more porous... and sea water impregnates the structure a bit.  The sea salt in the shell is another factor in the marking that the shells leave on the clay or glaze.  The sodium compounds in the shell pores volatilize and "flash" around where the shell is... adding coloration to the marks left.  Old beach collected shells are the best for effects.


Shells from fisheries don't get the 'knock around the ocean time' after the animal has died.  Seafood is shucked and the shells disposed of.  So they work well... but not as nice as the beach collected shells.


Shells from "commercial suppliers" .... as in 'buy them by the pound'....... usually are washed in fresh water during the processing work.  Not as good as fishery shells.  Not as much salt left.  But they work.


Typically, the shells are packed with another wadding material inside them.  The shells lose their structure in the firing... so if it is just a shell sitting under something...  the shell will collapse and lose its shape and let the piece over it tip/collapse/fall.  The wadding mix inside the shell is any typical wood firing wadding mix.


Now we come to "synthetic shells".  Yes........ a 'make your own' deal.  Some folks make different shaped wadding material that causes effects similar to shells in a wood kiln, but can be different shapes.  The standard mix used is a mixture of 1/2 calcium carbonate and 1/2 plaster of paris.  In the firing in the college's anagama that is cooling right now, one of my students is trying that, but I suggested he also test using salt water for mixing the material up instead of fresh water.   We'll see how that works out next Sunday.


To facilitate removing the shell from the fired work. pieces are placed into water and left for a while.  The shells and other wadding material soften and release.  Then some grinding with a Dremel to clean it up and .... done.


Note that the dust from the fired shells is not something to get into the air when unloading the kiln.


All that being said... I am speaking of firing to anywhere between Orton cone 9 and cone 14.  Have not tried it at cone 6.  Likely "less" impacts.





#125587 G 200, G 200 Hp And What Else.

Posted by JBaymore on 20 April 2017 - 08:56 AM

Thixotropy used in conjunction with clay body makes me smile. Another one of those pottery world expressions that I have yet to comprehend its use.


Get some poorly formulated cone 6 porcelain.......... throw with it with some added water.... don't keep anything........ put the pots and the throwing slop in a container.........let it sit a while.... reclaim it... and try to use it again on the wheel.  ;)





#125578 G 200, G 200 Hp And What Else.

Posted by JBaymore on 19 April 2017 - 10:21 PM

 I prefer shorter clay over thixotropic clay. 


Thixotropy in clay is a total no-go for me.  I'd rather work with wet beach sand.





#125488 Help With Substitute For Lead Bisilicate

Posted by JBaymore on 18 April 2017 - 10:21 AM

Yes, lead presents hazards, but if used intelligently they are perfecty manageable, the same as with many other potter's materials. And it is your choice whether to use it or not.


The potential issue with studio ceramics in this subject area is that many folks have not been given much accurate information about the subject of toxicology (or even glaze chemistry), and many also do not grasp the science and math concepts behind things like ppm, the impacts of significant figures, what orders of magnitude might mean to exposures, and the like.  This is one of the reasons that I teach ceramic toxicology as part of required technical courses in our undergrad program.  Everyone graduating with those BFA letters after their name gets at least an attempt to communicate some accurate information.  It is a start.


There is TONS of what amounts to 'hysteria' that surrounds the topic of health and safety in the studio.  Lead, manganese, barium, and copper seem to get the most 'high profile' hysteria.  Interestingly, the issues with respirable micro-crystalline silica seem to get a bit underplayed.


Another REAL potential issue with online forums is that a VERY wide range of experience level people read the postings.  So what is "good information" for some people reading it who might have the appropriate education and context... can actually be "dangerous information" to the less experienced.  And because those folks are "unknowns" as far as what their educational path is or has been...... you never know if they are going to get that important information that is "left out" in the thread they have read. 


Some read "Just spray XXXXXXX on the work and you'll get XXXXXXXXXX"........ and run off and do it with no further research as to what they are working with.  The "assumption" tends to be that it is art... so it is OK.


Yes, SOME people can work with lead in the studio safely.  And you CAN have fired lead glazed work that is perfectly safe for the consumer.  To do either....... takes some serious and accurate understanding of what the potential issues are and how to correctly solve them. 





#125367 Firing The Anagama This Weekend... Will Be Away

Posted by JBaymore on 14 April 2017 - 04:25 AM

FYI....... I'll not be around from about 5:45 Am today until Monday morning sometime.  It is time for me to supervise the spring firing of the college's anagama.







#125095 What Type Of Sand? To Add To Clay.

Posted by JBaymore on 08 April 2017 - 08:45 PM

A lot of sands are actually made up of weathered feldspar, granite, and the like.  Some contain weathered shells (as in calcium carbonate). 


Normal IS silica sand.  Coarsely weathered silica (flint/quartz).


At cone 9 range.... many sands that are NOT silica sand..... melt.  SOme with calcium carb tend to outgas...and casue "issues" or many sorts.  Calcium oxide (what calcium carb turns into... is a flux on silica.  SO a "melting agent".


The best answer is ............ test, test, test.


And remember that non ceramic sands are not tested for consistency for what you are doing with it.  So what is fine for other uses as far as variance goes....... may come back and bite you in the butt.





#124903 Throwing Away Rejects / Test

Posted by JBaymore on 05 April 2017 - 08:44 PM

Bury them deep in your back yard... in 1K years some archeologist will  go crazy over his or her find!!!


................and make erroneous conclusions about our culture.   ;)  





#124796 Qotw: Is It Craft Or Is It Art

Posted by JBaymore on 04 April 2017 - 10:53 AM

I just ignore it and make my art.





#124794 Pottery Knowledge Quiz Of The Week (Pkqw): Week 1 And Introduction/answers In...

Posted by JBaymore on 04 April 2017 - 10:19 AM

Pres said (heard a lot of erroneous information)

​This could be a question of the week

I have a few top add to that topic .


Oh yeah.  Erroneous information............. in online forums, in books, in magazine articles, in youtube videos, etc..  The list is long. 


The internet has made it worse than it used to be.  He or she with the loudest voice or the most repetition gets heard.  Quantitative over qualitative.


I always come back to the simple concept of "Vet your sources".  However, even that can be difficult.


I'll share something I talk about in my technical classes and have written in some handouts, that pertains to caveats about getting technical information.  This viewpoint is not "politically correct", nor will all like it.  But I think it is a very important subject to bring up with students.  Causes them to think critically.  (I'm re-writing this idea here... not copying the text from my handouts.)


There is this person whom we'll call, "Famous Ceramic Artist".  (Yes, I realize that "Famous" is a weird first name. ;)   Could be a man...could be a woman.  Don't know. Don't care.)  Famous Ceramic Artist makes STUNNING ceramic work.  Universally agreed amongst art critics and other ceramic artists alike........ this person is making some astounding visual work.  They exhibit in all the "right places".  They get featured in magazines.  They get pictures in books.  Heck, they WRITE books.  They do countless workshops "spreading their stuff".  They seem to be everywhere all the time.  They have throngs of "groupies" following them.  Life is good.


Remember... Famous Ceramic Artist became "famous" because of the nature of the art work they created.  BUT...... as a by-product of that fame, everything they say and do now becomes "ceramic gospel".


So when this person writes an article or a book or presents a workshop.... technical information usually gets shared. Potters love this stuff.  We are all about sharing the "tricks of the trade".  Maybe it is how kilns work.  How they fire.  Recipes for glazes and clay bodies.  How to fix defects in work.  Why clay is plastic.  How to control warping.  The list of possibilities here is long.  All of this information is taken as 'gospel coming from on high' also.  Because of course, they are "famous".


As Paul Harvey says ....... "and now for the REST of the story":


What you don't know....... and which does not in any way take away from the wonderful art work Famous Ceramic Artist produces... is that this MFA graduate of GKWU (God Knows Where University)......... got a D in their ceramic materials science class and also in their kiln building and firing class.  And that was a "Santa Claus" grade to boot.  You won't be able to find that information out easily or at all.  No one would talk about it....... because.... well.... they are "famous" now. 


So .... clearly stunning work execution......... technical information....... not so much.


It is VERY possible to be a total master of the things that you do in your own work, without having a broad based knowledge of lots of technical aspects of the craft.  We merely have to look at the amazing work of so many indigenous folk potters of days gone by,...... and know that they could produce amazing work.  But they knew little of the technical side of things outside their narrow area of expertise.  They were not trained in formal science.  And they might even be producing that work based on what that THINK they know... but making false attributions of cause and effect.  All they know is that when they do X........ they get result Y.  And result Y is what they want in the work.  It works to get the result ....... so it is all that is necessary to know.......and it must be true.


Sometimes even the Emperor has no clothes.


So.... again.......... vet your sources... carefully.  And look to MULTIPLE sources