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Grog in clay for throwing


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#1 Val

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Posted 10 November 2012 - 12:19 PM

I usually throw cone 5 stoneware without grog. I have recently moved from Southern Africa back to USA and by mistake got some clay with fine grog. I am at a loss how to smooth it, as a damp sponge just brings up the grog. B.T.W. Is grog needed in clay for throwing? The only way I could solve the problem was to make a sieved slip from the clay body, or burnish. How to others deal with this?
Is there a rule of thumb about when grog is needed? I've read it's a must for tiles, but what else?

I 'm sure my question is very basic and dumb, but I learned to throw in sort of a vacuum being in Africa and all.........
Thanks!
Val

#2 neilestrick

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Posted 10 November 2012 - 12:30 PM

Grog is not necessary, although it does improve the workability of the clay, plus reduces shrinkage, cracking and warping. You can make just about anything with any clay body, but grog makes it more forgiving. I just bisque fired 28 inch tall porcelain vase with no problems.....

To smooth a pots with grog, use a metal or rubber rib, not a sponge.
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#3 OffCenter

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Posted 11 November 2012 - 09:53 AM

Grog also weakens the clay.

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#4 JBaymore

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Posted 11 November 2012 - 11:03 AM

Grog also weakens the clay.

Jim


Wise man. MOR tests bear this out.

While it can improve the plastic throwing/forming qualities of some clays, and can help with potential drying issues by letting moisture more easily migrate through the walls, and even out/decrease shrinkage helping to prevent things like warping, grog is a double-edged sword.

The shrinking clay surrounding the already fired grog particles sets up a network of micro-cracks surrounding the grog pieces that form the basis for later failure under stress. This process starts in the wet to dry shrinkage, and continues in the dry to fired stage.

best,

.................john
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#5 neilestrick

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Posted 11 November 2012 - 02:10 PM

Grog also weakens the clay.

Jim


But not to the extent that it makes your ware too fragile for daily use. If you need it, use it.
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#6 AnnaM

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Posted 31 January 2014 - 12:21 AM

Hi guys, just a real newbie question on this issue- if you are making your own bisqued clay grog for a clay body, does the grog have to be made from the same body? For e.g. porcelain for porcelain, earthenware for earthenware?

#7 Biglou13

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Posted 31 January 2014 - 01:04 AM

No
But molochite in white or porcelain bodies is nice.

The christy 20-48 is what I use along with larger screen, and sometimes larger particles.

Grog from same body is a treat are you going to ball mill?

I've even used combination of different grogs in one clay.
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#8 Babs

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Posted 31 January 2014 - 01:58 AM

Is it right that grog allows clay to endure thermal shock, better if hte pots are to be used for oven ware??



#9 AnnaM

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Posted 31 January 2014 - 06:11 AM

Biglou, I have no idea at the moment how I'm going to make grog! It's been suggested that it would be possible to smash the bisqued clay up inside a heavy duty bag of some sort using a tenderiser or some type of hammer and then sieving, or grinding with a mortar & pestle (and then sieving). I'm not talking large quantities, just experimenting at this stage!

#10 Biglou13

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Posted 31 January 2014 - 07:52 AM

Milling your grog isin't that fun or easy.

Grog size also plays some importance. So screening may be necessary.

Sometimes silica sand can be used a grog, cheap and easily available.

I really like the feel, and like to work with groged clay. I will usually just add when wedging.

I've even added what some would consider pebble size. (Chicken grit)

If I were you I'd just buy some med and or fine grit to start with.. And see if groged body agrees with you.
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#11 Pres

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Posted 31 January 2014 - 11:18 AM

I have thrown a lot of different clays, some with and some without grog. I used to like the grog for tooth and strength when throwing larger, but have found over the years that most of the finer grogged  or minimally grogged clays work just as well for me.  Extremes don't do a whole lot, 50% grog as in Penn States raku clay during Don Tigney years was brutal-open sores all summer long. At the same time throwing a fast drying raku that had 35% perlite was skill taxing. Imagine stretching a belly wall out with that stuff in the clay!  So now my clays are pretty smooth and plastic from SC.


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#12 Mart

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Posted 31 January 2014 - 04:04 PM

Fine grog is good for you. Why even worry about it? How can this be a mistake? You used wrong "tool" to smooth the surface :)
 
If you get clay with >2 mm grog, this starts to be a tricky stuff to throw because it's really hard on your hands. Ever fallen down on coarse asphalt and got your palm all scraped? 
 
 

...
The shrinking clay surrounding the already fired grog particles sets up a network of micro-cracks surrounding the grog pieces that form the basis for later failure under stress. This process starts in the wet to dry shrinkage, and continues in the dry to fired stage.

best,

.................john


Hmm... this sounds interesting, especially the "continues in the dry to fired stage".
Can you elaborate on this particular phenomena?

#13 Biglou13

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Posted 06 February 2014 - 10:45 PM

Fine grog is good for you. Why even worry about it? How can this be a mistake? You used wrong "tool" to smooth the surface :) If you get clay with >2 mm grog, this starts to be a tricky stuff to throw because it's really hard on your hands. Ever fallen down on coarse asphalt and got your palm all scraped?

...The shrinking clay surrounding the already fired grog particles sets up a network of micro-cracks surrounding the grog pieces that form the basis for later failure under stress. This process starts in the wet to dry shrinkage, and continues in the dry to fired stage.best,.................john

Hmm... this sounds interesting, especially the "continues in the dry to fired stage".Can you elaborate on this particular phenomena?

Here is a picture I'll let sensei John explain. (Plus not sure how to explain it). But there were cracks in bone dry stage and even bigger in, fired. I look to refire these .
A picture is worth a lot o words.
(3 day woodfire, self made clay, granite inclusions, aka chicken grit, first firing)

http://community.cer...age/3712-image/

Was unable to resize to post
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#14 Norm Stuart

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Posted 07 February 2014 - 12:29 AM

I do hope you know you can inexpensively purchase "grog" from your clay supplier.

 

It comes in a variety of mesh sizes, and you have a choice of what material the grog consists of.

 

http://www.axner.com...hite-50-80.aspx

http://www.axner.com...e-100-mesh.aspx

http://www.axner.com...te-48-mesh.aspx

 

We have Kyanite/Mullite on hand to add for the thermal shock resistance it adds.

 

I can't imagine grinding and sieving my own grog - nothing worse grinding up over-fired pieces and realizing you've added cristobalite grog to your clay.

Biglou, I have no idea at the moment how I'm going to make grog! It's been suggested that it would be possible to smash the bisqued clay up inside a heavy duty bag of some sort using a tenderiser or some type of hammer and then sieving, or grinding with a mortar & pestle (and then sieving). I'm not talking large quantities, just experimenting at this stage!



#15 AnnaM

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Posted 07 February 2014 - 01:46 AM

Yes, but unfortunately I want it coloured and there's nowhere in Melbourne I can get coloured grog! (other than red/terracotta coloured)

#16 AnnaM

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Posted 07 February 2014 - 01:49 AM

It hasn't been that bad making it actually! Just messy, but fun smashing up the larger bisqued pieces initially ;)

#17 Mart

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Posted 07 February 2014 - 11:55 AM

Fine grog is good for you. Why even worry about it? How can this be a mistake? You used wrong "tool" to smooth the surface :) If you get clay with >2 mm grog, this starts to be a tricky stuff to throw because it's really hard on your hands. Ever fallen down on coarse asphalt and got your palm all scraped?

...The shrinking clay surrounding the already fired grog particles sets up a network of micro-cracks surrounding the grog pieces that form the basis for later failure under stress. This process starts in the wet to dry shrinkage, and continues in the dry to fired stage.best,.................john

Hmm... this sounds interesting, especially the "continues in the dry to fired stage".Can you elaborate on this particular phenomena?


Here is a picture I'll let sensei John explain. (Plus not sure how to explain it). But there were cracks in bone dry stage and even bigger in, fired. I look to refire these .
A picture is worth a lot o words.
(3 day woodfire, self made clay, granite inclusions, aka chicken grit, first firing)

http://community.cer...age/3712-image/

Was unable to resize to post

 
If you got visible cracks at try stage, you will get bigger cracks after the firing. There is no magic involved in this, grog or no grog. This is how clay behaves.

What I was curious about, are those micro cracks, John was talking about. Maybe it's a question of definition? 
When most of the added water has evaporated, dry clay is one big pile of "micro cracks" :)
As little as I understand, those "micro cracks" vanish in firing thanks to vitrification.

Biglou13, that little plate looks awesome. Refiring will ruin it for sure.




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