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How Much Grog to Wet Clay for Raku


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How much grog is best added to B-Mix for use in Raku.

Answer = what percentage, by weight, is an appropriate amount? 10%, 20%, 35%, 50%, ?????????

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99% of Google hits are about adding amounts to raw materials to make a clay body. I put all the values I found into my black box and the result was 20% to the question above. I think 20% is too much but I don't know. I do know it took a heck of a lot of wedging to get a pound of grog into 5 pounds of clay and by appearance and wedging behavior it seems like its way too much.

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Sounds about right, depends on how quickly you are heating and cooling the pots etc. Fine or medium grog? If you guesstimate the wet clay having around 20% water you will be a bit over your 20% grog target but it should be close enough. Have you tried cut and slam wedging to get the grog mixed in? Video below if you need it. When I've added grog (or sand) to clay I make a rough block shape with the clay, slice it up and sprinkle the grog on each layer then slam them together. Add water with a spray bottle as you need. Once the grog is layered into the block then do the slice and slam wedging to get it mixed in. 

https://youtu.be/HApNjUnI9U4

Edited by Min
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Thanks Min. Here's how I do it:

- roll out five pounds of clay

- spread a layer of grog (Kyanite 35 mesh)

- roll that up, slice and slam wedge

- repeat until all the grog has been added

- spiral wedge, then to the wheel

 

One thing that makes me think 20% (1lb grog to 5lb clay) is too much is B-Mix 5 with grog from the store doesn't' look at all like my B-Mix with 20% grog.

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I haven’t, even though I do have some. I just got started with Raku so wanted to do all my learning curve stuff with some reclaim I have which is a scrap mix of B-mix 5 w/wo grog, some Buncombe and other ‘whites’. There’s not a lot of b w/grog in it. So far what I’ve used without adding grog has done well with few cracks; only one explosion so far. :rolleyes:

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On 9/17/2021 at 4:11 PM, Min said:

I believe B-Mix with grog is going to be much finer than 35 mesh. I'm guessing it's more like an 80 mesh. 

I’m betting you are right. I tried to find out on Google but couldn’t find B-Mix specifications that went that deep. Searching Google for “what mesh grog for raku” the prime hit is 20 mesh. Couldn’t find that locally so went with 35. I’m going to cut back from 20%, my gut is telling me that’s just too much. Will try 10% and 35 mesh and see what happens.

Thanks for the responses Min!

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  • 3 months later...

You are much better off using a raku specific clay that is designed to mature at cone 04-06 than using a cone 10 or cone 5 clay that you add grog to or comes with grog. 

Your pieces will be much stronger that way. I used to raku with a cone 10 clay called WSO.  It was recommended by my raku class instructor. Had two large beautiful platters break in my hands when holding them by the rim like you hold your plate at a buffet. Swore to never ever use a cone10 clay for raku again

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  • 5 months later...
On 1/17/2022 at 1:41 PM, Started Late said:

You are much better off using a raku specific clay that is designed to mature at cone 04-06 than using a cone 10 or cone 5 clay that you add grog to or comes with grog. 

Your pieces will be much stronger that way. I used to raku with a cone 10 clay called WSO.  It was recommended by my raku class instructor. Had two large beautiful platters break in my hands when holding them by the rim like you hold your plate at a buffet. Swore to never ever use a cone10 clay for raku again

You realize that all “raku” clays that are sold are in fact cone 10 stonewares, right? They absolutely do not mature at cone 04-06, you do not want a clay that matures/vitrifies at such low temperatures because it will not be porous enough for smoke from the post-firing reduction to work at all. Most suppliers take a white or off white cone 10 stoneware they already have and add a little more grog or sand to it, if you are lucky they might put in some kyanite or pyrophyllite to really help with the thermal shock. Lots and lots of professional raku artists use various cone 10 or even cone 6 stonewares that are not “raku” clays and produce wonderful work, some, like me, even use porcelain paperclay in raku and manage quite well with it. Some people prefer to not use an advertised raku clay because they may be rougher on the hands if they are doing a lot of wheel work, or have too much grog and sand that when burnishing the pieces prior to doing naked raku the grog will come to the surface or sand will fall out in little chunks, marring the extremely smooth surface they are going for, so they may prefer a stoneware with less grog and no sand.

 WSO is extremely well known and well regarded in the community for its use in raku, while I refuse to use Laguna clays ever again, it is extremely doubtful that your platters broke because you were using WSO, platters are some of the toughest shapes to get to survive raku just like it is very hard to keep large (or even small) platters from warping and cracking in regular firings,  it is extremely hard to heat them completely evenly in the kiln which means when you take them out they are more at risk of breaking from thermal shock including during the post-firing reduction, I would guess that too fast and uneven cooling was the problem with them and there is also a sweet spot with raku for platters or slabs in terms of thickness that is very important as well. Your clay was not the problem. It is perfectly fine to use a stoneware of just about any kind for the most part as a raku clay, and it comes down to individual preference if they need to even add grog or sand to it, and if they do how much, what kind, what size, etc. It can be a trial and error period in the beginning until they dial it in. I use porcelain paperclay primarily with some “raku” clay that I like because it is really really white, but I am developing my own recipe for a white stoneware and one of my planned uses for it is in raku, pit, and saggar firing, sometimes probably as paperclay, other times not, and sometimes with porcelain mixed with it. It will have only a very small amount of molochite in it but I will be subbing a good portion, if not all, of the silica with pyrophyllite so it should do well with thermal shock a lot more than using a lot of grog would provide. 

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I've used both WSO and Soldate 60 for Raku with great success and have pretty much settled on the WSO because the Soldate 60 has a pinkish tinge to the final product while the WSO is more white...both are ^10 stoneware clays.

Edited by JohnnyK
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