Removing tool marks - Hydrochloric Acid

["Pete Hinde" <Peter.Hinde@xxxxxxxxx>]

Thought I might pitch in here with a hope to clarify some chemistry

Marble (calcium carbonate) will react with hydrochloric acid to give calcium
chloride, carbon dioxide and water.

Calcium chloride is a very soluble salt of calcium and has the property of
absorbing water from the atmosphere. This property is termed deliquescence
and would explain why the stone appears to be wet for ages if it isn't
washed off. (in fact calcium chloride is occasionally used as a drying agent
because of its ability to absorb water)

I have already seen a response to this message with reference to the fumes
(I would have described them as white). These would be fumes of hydrochloric
acid (Hydrogen chloride)itself and would explain why steel would be
susceptible to corrosion.

I dont believe that chlorine gas is a product here- (and chlorine gas is not
mustard gas) although the acid fumes are unpleasant enough in themselves.

What really prompted me to write this was the mention of hydroflouric acid
(in a response to this letter). This acid is in a different nastiness league
to hydrochloric acid (check out eg
http://www.emedicine.com/emerg/topic804.htm and to mention its use casually
is to miss the strict precautions that would need to be taken when dealing
with it. Whilst I have been fairly blase about my dealings with HCl I would
never use hydrofluoric acid unless I really had to..and only with extreme
caution.

I have always been intrigued as to the statement "wear a good dust mask".
I'm still not sure what makes a good dust mask..nor of a recommended make
(any comments here?). Needless to say the mask you might choose to wear
whilst dealing with acids won't be the same.

sorry if I've carried on a bit here...I'll pipe down..regards..pete