[Rockhounds] Hydrochloric acid use

[Rock Currier]

Cynthia,

Some of the information you have received from members of the chat group is 
not accurate and may not be very helpful in cleaning your raw rocks. The 
first step is using acids to clean minerals and rocks is to understand what 
it is you want to clean. What are the nature of the "raw rocks" you wish to 
clean? Are they quartz crystals?  Did someone tell you that you needed 
hydrochloric acid to clean them? Some carbonates like calcite will react 
with hydrochloric acid and dissolve in the acid solution and generate 
bubbles of carbon dioxide during the process.



Much of the time this acid is used in conjunction with quartz and quartz 
family minerals to remove brown iron stain (iron oxides and hydroxides). Is 
this what you are trying to achieve? You can often clean away iron stains 
with hydrochloric acid, but you usually need to use it in its concentrated 
form for most effective cleaning. There are also a couple of other methods 
to achieve this that involve less reactive chemicals, though one of them is 
poisoness if you ingest it.



If you want to clean quartz or just rocks, the first thing to do is to clean 
them as well as you can by scrubbing it with soap and water. If you have a 
quantity of them, a quick trip to one of those your wash your own car places 
to use their high pressure water guns may be helpful.



If you still want to use your acid to clean your rocks use rubber gloves 
when handling the acid and use some sort of eye protection to prevent it 
from possibly splashing in your eyes. Do your work outside in a 
well-ventilated area with a garden hose handy to wash up and dilute any 
spills. If you get some on you, don't panic, just pick up the garden hose 
and flush the area of contact several times to dilute the acid to a point 
where it is harmless. If you do spill some on you and don't notice it, 
likely the first think you will feel will be an itching sensation. This is 
usually the tip off that you have gotten some acid on you. Flush the area 
with water to get rid of the acid. If you still think acid may be present 
you can sprinkle a little bicarbonate of soda on the area. If acid is 
present it will bubble. I often just flush the area with water and taste it. 
If you taste something sour, acid is still present. A little taste of dilute 
hydrochloric acid will not hurt you at all. It is generated naturally in 
your stomach to help you digest food. Do your work in an area where children 
cannot reach it. Don't do it in your garage. Fumes from your hydrochloric 
acid will react with many things in your garage, especially if you leave it 
there a long time. A five gallon plastic bucket is a good container to use. 
Place your rocks in the bucket and just cover them with acid. Don't fill the 
bucket any more than about 60 to 70% full. The concentrated acid will give 
off a strong smell (hydrogen chloride) and you may find it better to place a 
lid of some sort on the top of the bucket. It will usually take a day or 
more to remove any rust stains from your rocks. It is a function of how deep 
seated the iron stains are, the strength of your acid, and the temperature 
of your acid solution. The warmer the solution, the faster the reaction will 
take place. Do not however heat the acid up on your stove. If you want to 
increase the reaction rate of the solution, place the bucket in the sun and 
wrap some black plastic around it. This will warm it up nicely. The reaction 
rate will approximately double for every ten degreed centigrade increase in 
temperature. Take a look at your rocks the next day to see how they are 
comming along. In most cases you will notice a noticeable decrease in the 
iron staining.  The first part od the cleaning goes pretty quickly but you 
may have to leave it in the solution for several days to get most of it out. 
It is the last little bit that usually takes the longest. If the iron stain 
is really deep seated, for example located deep in cracks inthe rocks or 
enclosed completely in the crystal latis of the minerals, you man never be 
able to get it out.



After you remove your rocks from the acid solution you should wash them well 
with plenty of water and then place them in a bucket of water and let them 
sit for a few hours or over night. You may need to repeat this procedure 
several time to make sure that all the acid has been removed from your 
specimen. If you are cleaning pure well formed quartz crystals they will not 
need much more than just a quick rinse, but if you are cleaning up some 
porous rocks, like microcline feldspar or granite, you may need to cycle 
them in and out of clean water a number of times to make sure you have 
removed as much acid as possible. After you have cleaned your rocks, you 
will need to close your bucket up tightly to save the acid for further use 
or you will need to dispose of  the acid. If there is not very much acid you 
can probably get away with diluting it away with water. To effectively 
neutralize it you should react it with marble chips (calcite) till the chips 
stop bubbling. At this point you will be left with an innocuous solution of 
calcium chloride and marble chips.



Yours truly, Rock