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Today we have a different take on the PQotW, as I got a message from LeeU:


I took a whack at making a Pottery Knowledge Question of the Week for future considerfation. No offense taken if it is not suitable or ever used. Discovery: it ain't that easy to do! Didn't do an intro-just the Q & A.


 


The topic is from Clay and Glazes for the Potter, Daniel Rhodes, revised edition, 1973, Chilton.   Color in Glazes and Slips  


 


1. The color of fired clay is affected by:  (pg. 43)


 


  1.  The presence of iron in the body;
  2. The atmosphere of the kiln;
  3.  The temperature of the firing;
  4.  All of the above.                                                                 

 


2. The color in glazes may be due to:  (pg. 205)


 


  1.  The color of the clay, slip, or underglaze;
  2.  Metallic oxides;
  3.  Overglaze enamels and lustre films;
  4.  All of the above.

 


3.  Red glazes are produced with: (pg. 213)


 


  1. Iron Chromate;
  2. Uranium oxide;
  3. Cadmium and selenium;
  4. Rutile.

 


4. Colored slips (engobe) should be applied to:  (pg. 250)


 


  1. Fairly wet clay;
  2. Leather hard clay;
  3. Bone dry clay
  4. Bisqued clay

 


She even gave me the key to this so will hold that til the usual time.


 


best,


Pres


 


Answers:


Sorry folks, I have been looking for my copy of Clay and Glazes for the Potter, but it must be packed away(we are redoing the library soon). So I will post Lee's answer key:  1-4; 2-4; 3-3; 4-1 I wish I could have added in the text surrounding where the questions came from.


 


best,


Pres


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4 - all of the above

4 - all of the above

3 - cadmium and selenium   (out of the choices given)

1 - fairly wet clay    

 

(when I went to school I was taught an engobe contains 50% or less clay, a slip has 50% and over. now it seems the terms are interchangeable so I went with 1. If it’s an engobe in the classic sense of the word then it would be 2,3 and 4 depending on the percentage of non plastic ingredients in the mix)

 

Good questions Lee  :) 

Judith B likes this

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Rhodes wrote his bible in 1957...maybe the evolving slip/engobe terminology was missed in his '73 revision or hadn't shaken out yet. It was fun trying to find 4 single-subject questions in a cited source, plus 4 fairly reasonable answers with the correct one(s) located in the cited source. But not easy, at all! I encourage others to try it! 

Pres likes this

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4

4

3

2

        I think #4 was a tricky question. I picked #2 because I have seen a friend of mine back in the days when I was in College. She applied it at the leather hard stage. But I think it can also be applied at the bisque and bone dry period. But IDK about wet clay. Maybe if it is not too wet. But hey, sometimes you gotta break the rules and try it out. I haven't, but I bet someone did. Could have interesting effects, maybe. Aw man, now I gotta make a engobe. Soon as the heat here in South Texas gives me a break. 

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My best guess since I don't have this book you mention:

 

1 = 4

2 = 4

3 = hmmmmm (I am not the glaze type)... hmmmm... let's say no. 1 (Iron for sure, but the chromate is green no?!)

4 = easy-peasy: no. 2

 

Happy Sunday to all!

 

Evelyne

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I think there may be some opportunity for further discussion around question 4.  I interpreted the question to be about engobes since the term was specifically used. 

 

I don't have a copy of Rhodes, but, per the answer to question 4, it seems like he's treating slips and engobes the same (except that engobes include colorants).  However, other resources such as this CAD post from Robin Hopper make a different distinction between slips and engobes:

 

Whereas the simple liquefied slip commonly is used to coat greenware, an engobe can be formulated for use at any stage, including over bisque-fired ware. Engobes also often are used without a covering glaze, giving a wider potential for experimentation with the surface. An engobe or underglaze is more like a glaze in structure and may contain very little plastic clay.  

 

https://ceramicartsnetwork.org/daily/ceramic-supplies/ceramic-raw-materials/slip-engobe-or-underglaze-robin-hopper-demystifies-three-common-pottery-materials/

 

Sometimes sources will note that engobes have less clay than slips, often contain frits and have lower shrinkage rates. I've misplaced my copy of Hamer, so if someone could post the definitions from there it would be greatly appreciated.  The answer to this question "Fairly wet clay" would have the highest shrinkage rate.

 

I'm not sure that it ultimately matters - it's more whether the slip or engobe works with what you are trying to achieve in your work.  But it is kind of fun having these questions each week.  I think they are great learning opportunities, and I look forward to Pres' weekly post.  Thanks for taking the time to do this.

 

-SD 

Judith B likes this

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I totally agree with your assessment of the text. You and many would argue the point with you, as I believe when the Rhodes was written there was much that had not been established about slips and engobes, as the understanding of chemistry has changed. I will revisit my Rhodes when I find it. Right now so many of my texts are stored in the attic while we get ready to redo the library/guest room. Takes a long time to pack books, and we are still packing!

 

 

Thank you for the kind words about the PQotW, it is my intention that it raises the types of questions you have posted, and that all of us, including me get a refresher, or an update on our understanding of all things Ceramic.

 

 

best,

Pres

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If you think of Marbled English Slipware decoration or Steven Hill’s method of applying slip it goes on either fresh slabs or freshly thrown pots while still on the wheel and it’s made from a claybody, sometimes deflocced or coloured, not from a distinctly formulated engobe. Since there was only room for one answer and engobes can fit leatherhard, dry or bisque then wet clay had to be it. The word “slip†wasn’t in brackets in the question so I took that to be the significant word. Agree with you both that a clear definition of slip versus engobe would be helpful.

S. Dean and Pres like this

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4-4-3. (4)- 3 or 4

Uranium ox produces yellow/ silver. Makes your clay glow:).

Imagine my angst in my early years as a young art teacher reading about enamels for copper enameling and the hazards of many of these powdered glasses which we were using in our Jewelry and Metalcraft classes. Then I read about the common yellow components for enamels sold in the 60's and early 70's. Went to the Science department and picked up a Geiger counter only to find that the needle went off of scale on one entire shelf of enamels. We had the Science people get rid of them. The things we didn't know, and the way we trusted manufacturers to supply safe materials. Some would say you are responsible to know these things. .. I say that we were responsible to keep informed, not necessarily know everything.

 

best,

Pres

Marcia Selsor and Min like this

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