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Grog in clay for throwing


Val

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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

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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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Guest JBaymore

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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  • 1 year later...

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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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!

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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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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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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?

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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.ceramicartsdaily.org/gallery/image/3712-image/

 

Was unable to resize to post

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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/molochite-50-80.aspx

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

http://www.axner.com/kyanite-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!

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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.ceramicartsdaily.org/gallery/image/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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  • 2 years later...

Hi all,

 

I am fairly new to pottery but wanted to try throwing different clay bodies in various difficulties. One thing that really fascinated me are high grog content clays. The coarser the better. I came across this moon jar that I wanted to attempt but wasn't sure how I could come up with it. The artist said it is a mixture of porcelain and abereiddi stone + white jun glaze. Can someone advice on how I can get this clay body?

 

http://www.adambuick.com/shop/miniature-moon-jar-50/

 

Any help is greatly appreciated.

 

Many thanks,

Jason

 

 

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John:

Ron and I had a lengthy conversation about the advantages and disadvantages of grog and grog varieties. Mullite is commonly used as a grog, although it is highly calcined kyanite. Mullite does help with thermal expansion/shock issues, but in and of itself is not that good overall. Kyanite however will grow long needle like formations in the clay which actually does add strength to the body: without the water absorption when forming. The draw back to kyanite is the temperatures required to get those needle formation to occur. In cone 6, 200 mesh kyanite is more desirable along with stronger fluxes to get the needle formations: however those will be limited. In cone 10, 50 mesh kyanite would work. Currently I am using 3-5% 200 mesh kyanite with additional calcium fluxes to expedite a better melt. Not a fan of sodium for many reasons: being very cheap is not on my list of considerations.

 

Mullite morphology also is important for its application. In this case, there are two common morphologies for mullite. One is a platelet shape with low aspect ratio and the second is a needle shape with high aspect ratio. If the needle shape mullite can form in a ceramic body during sintering, it has an effect on both the mechanical and physical properties by increasing the mechanical strength and thermal shock resistance. The most important condition relates to ceramic chemical composition. If the silica and alumina ratio with low basic materials such as sodium and calcium is adjusted, the needle shape mullite forms at about 1400 °C and the needles will interlock. This mechanical interlocking contributes to the high mechanical strength of porcelain.

You have to start with kyanite to get these needle formations: will not happen with mullite.

Nerd

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Is it the speckle you are looking for?

 

you could make your own grog,  from either calcined or bisqued or even fully fired clay of your own   id start with coarse manganese dioxide additive and some cobalt if you want that look........

 

if your question is about grog......heavily groged bodies    just throw grog in the clay  as much and as big as you can and the clay will tolerate.  youll be surprised.......

 

google grogzilla

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Hi all,

 

I am fairly new to pottery but wanted to try throwing different clay bodies in various difficulties. One thing that really fascinated me are high grog content clays. The coarser the better. I came across this moon jar that I wanted to attempt but wasn't sure how I could come up with it. The artist said it is a mixture of porcelain and abereiddi stone + white jun glaze. Can someone advice on how I can get this clay body?

 

http://www.adambuick.com/shop/miniature-moon-jar-50/

 

Any help is greatly appreciated.

 

Many thanks,

Jason

 

Porcelain is porcelain.  The key is his addition of a local stone -- abereiddi -- which he wedges into the porcelain and then fires.  The stone is likely a high iron content stone, so it shows through the white jun glaze.  Basically, he adds the local stone to his clay.  Whether he mixes his own porcelain or buys a commercial porcelain -- unknown.  But he adds the stone.  Likely fired to cone 10 or hire to get the stone to melt properly. 

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