Ian wrote:
From: stuart noble
Date: Fri, 10 Sep 2010 Time: 19:42:28
Not sure why the acid would have produced a brown colour
It's the action of the acid on ferrous fragments in the stone. See the
para beginning "York Stone" in:
http://www.pavingexpert.com/maintain_02.htm#acid
and the para headed "Concrete Paving" under the pictures.
I cam assure you that all my cheap indian sandstome was washed with
brick acid, and no stains resulted. Huge algal blooms did, however.