Good question. I missed the cotext in which it was posted, but maybe it's just a rock used to smooth over areas that are too rough??

Rub brick?

Here is a link that might be useful: {{gwi:70011}}

I don't know, but some of us use broken pieces of tufa as tools to smooth and shape our pieces. Sometimes we even make tools of tufa just the right size and shape for this purpose. Maybe something similar?
Jo

Tufaenough, thanks for the rub brick pic, looks like it's good for hard cement mix.
Gottatufa, does this rubbing of Tufa with Tufa just rough up the surface, or does it smooth it up?
Slate

Hi Slate;

I use a broken bit of concrete slab to smooth the upper edges of my containers when they're about two weeks old. The piece I use is bumpy and rough on the useful side. Then, I wire brush (which makes barely a mark), and hose off with lots of pressure but little water. I use the edge of a garden trowel to get most of the bumpiness out when the containers are first unmolded, and are still quite green and fragile.

Blooming

Yes, tufa made tools are really nice for smoothing, shaping etc. We had a post on here once about our favorite tools and alot of favorite tools involved tufa shaped into tools. Our "bump" fairy missed this one for us.
Jo

I hear the "bump" fairy is semi-retired from reading the last page. Seems like it doesn't pay enough for all of the abuse--typical corporate job! New volunteers are welcome to join in the fun!! :)