kaolin and other additives are added to zisha to enhance plasticity nowadays that they can be wheel spun easily. through blending between zisha clays, i'm not sure if plasticity would be enhanced.
was niangaotu ever slip-casted? i've yet to see an example but would love to. there are many theories on what niangao is. attached is one of the more credible explanations of this material
in principle, the clay blend that was milled for slip-casting between '58 and 60 was the finest particle size clay ever in F1, even finer than nian-gao. but neither niangao nor this 58-60 hong ni ever fired up to zhuni texture......
zhuni appears to remain as a percentage component than an absolute component. these are the various groups/grades that appear to exist
1) creamy jade like textures : players of ming to qing tomb excavated/robbed zhuni only, mostly small items. strangely, many of these pots have excellent fits, shape, and little warp. this led a belief in a size limitation of zhuni. many of these pots are very delicate and thin walled, clay is very smooth and appears less blended
2) thai zhuni with brass handles, these zhuni takes on an orangey to red hues, seldom burgundy. texture is less jade like, still v smooth but the pots are huge, 500ml and even more. walls are thicker
3) pear skin, apparently blended with post fired ground particles
4) wet tomb excavated, the zhuni's firing has receded and pot appears orangey/very porous
5) modern electrokilned zhuni, thick walls, glossy and occassionally creamy
6) taiwan driven zhuni replicas
Well..Zhuni may not be able to be thrown on wheels, but so is Duanni, Zini etc..matter of fact, Zhuni not plastic is quite a new word to me..not seen from text books or Zhu Ze-wei's that clay book.
Usually, sand type Yixing clays are called unplastic like Di Cao Qing, Duan Ni, Lv Ni etc so Pin Pei (mixing clay) or storing (aged clay gets more plastic) is often done to these clays to enhance plasticity. Think about Nian Gao Tu, which was made to play in slipcasts, was the most Zhuni-like Factory-1 clay with fine particles and smooth.
Zhuni was often made very little and thin without mixing, none-plastic clays can't be made this way..but I'm no potter..