This may be the first time some people have seen this material (top picture). Rice hulls, also called rice husks is the outer covering of the rice grain. Once this is removed, you have white rice. It took me a while to track this down. Finally I found it at a local feed store. It comes in a 25 lb. bag, is used as bedding for horses and it not expensive.
For my hand-pinch bowls, I wedge the rice hulls into my paperclay. How much to add? If you want more texture, add more rice hull. Use less for a subtler effect. There should still be enough paperclay to hold everything together. You will know it if what you make does not hold up. In that case, add in more paperclay and wedge till the rice hull is evenly distributed in the paperclay. You can also press the rice hulls onto the outside surface of your form but this will give a different kind of look/texture.
This material gives a VERY delicate texture (the second picture is the couples cups on its stand, the bottom picture is a close up shot of the texture) so I opt to use oxides instead of glazes. A heavy application of red iron oxide give a very grayish gun metal color in Cone 10 reduction firing. I often use this oxide to contrast against a lighter colored glaze which I use for the inside of my hand-pinched forms.
In addition to creating interesting textures in paperclay, you can also use the rice hulls as a organic combustion source for your raku post-firing reduction.