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On 6/17/2018 at 6:51 AM, glazenerd said:

Babs:

sodium bentonite absorbs up to 15 times it's weight in water. Ball clays absorb water as well, but that is expressed as WOPL. (Water of plasticity). Example: 100 grams of OM4 ball clay requires 35 grams of water to produce a pliable ball. When sodium bentonite ( not calcium bentonite) is used as a plasticizer in clay: it only absorbs 1/3 to 1/2 of the water it can actually hold. Ball clays do the same, but not on the scale of bentonite. Then when you throw, both continue to absorb water: which in turn weakens the mechanical properties of the body.

point 2: potters tend to keep adding ball clay or bentonite to their clay recipes so that it comes out of the pugger all creamy, dreamy, and plastic. The problem with that is 1: the higher the % of bentonite/ ball clay used to produce that result: the moe water it will absorb when you throw it: resulting in mechanical failure- slumping walls. CEC ( cation exchange) is the chemical reaction that results in plasticity. CEC has a rate law: so much exchange per 24 hour period until maximum exchange takes place. Usually 3-5 days. I use a ball clay with a CEC of 11.0 at 8% of recipe: which produces more plasticity than 15% of OM4 which has a CEC of just over 7. However, it is short when it comes out of the pugger, but five days later it is plastic. Not FAT, but plastic. 

A simple way to judge excess  plastizer is the amount of cream on your hands when you throw. If it creams and oozes through your fingers, way too much plastic materials. Which also means it will get weaker the more you wet it and handle it on the wheel.

point 2. The finer the particle, the higher the green strength. Ball clays with particle sizes under 0.50 microns, have MORvratings around 700. MOR= modulus of rupture: a testing method used to determine green strength. As particle sizes increase, MOR values decrease. Porcelain is known for its fragile green ware state: but kaolin runs 1.4 to 2.5 microns typically. ( there are blends lower and blends higher in PSD.

lastly, if you are finding white fibrous patches or granular areas in your dried reclaim: I would put Nep Sy as the flux used in the body. Nep Sy has 14-20% soluble salts that do migrate as the clays dry. Ten plus years ago when clay used Minspar, G-200, or other potash as flux; pinhole issues where fewer and farther between. Now that Nep Sy is the body flux of choice; pinhole issues are common.

t

Can adding a scoop of Dry Mixed clay to a batch of recycled clay return it a bit to a stronger (purer) clay?

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