limestone cleaning tips

[Will Shotton <shottonmasonry@zzzzzzzzz>]

  Hi,

I've never used bleach to clean limestone but I have often used a light
to moderate muriatic acid solution masonry cleaner available at most
masonry supply shops. After removing the majority of the old Portland by
hand (plugging chisel + mash hammer) you scrub the entire surface with a
stiff natural bristle brush and then rinse with lots of water. Repeat
until you're satisfied or no further improvement is noticeable. This
method is labour intensive but is the least damaging to the delicate
limestone. Harsher methods like sand/soda blasting and high pressure
water can do a lot of damage really quickly and accelerate the aging
process. An over-eager sandblaster is a loose cannon. I specialize in
heritage restoration of valuable stone structures and have come across
some pretty ugly attempts at so-called restoration jobs. Least invasive
methods first, I say.

Just a note: careful using the plugging chisel and hammer. Like carving,
send the force into the stone when you're near the arrisses and corners
to prevent knocking chunks off of the valuable original materials.
Careful use of a grinder and diamond blade to score the old Portland
down the middle of the joint and then knocking the old Portland out
works well.

When I'm asked to use a proper heritage lime mortar I use a  6:1:1 mix.
6 sand, one autoclaved mason's lime and one White Portland. The white
Portland is a much purer portland which will not lead to efflorescence.
It also reduces wait times for striking the joints. If you want a softer
mortar increase the lime content and reduce the portland
correspondingly. This 6:1:1 mix will give a much lighter, whiter joint
when it sets but so would a pure lime based mortar. The sand selection (
use sharp sand ) will also affect the colour. The compression strength
of the lime/portland mix is less than most building limestones so you
won't risk damage to the masonry units.

Good luck! Oh, and use goggles, gloves and don't expose skin to the
muriatic acid--for obvious reasons.