cleaning rocks

[MALACHI]

Help required in advising me on how to clean some rocks that I've recently acquired. The two I very much would like to get spotless ( if possible ) are nice specimens of acicular wavelite and  prehnite. Their surfaces appear to be stained with a calcium or iron oxide film. So far without success, I've tried warm soapy water with a tooth brush.   At this stage I'm baulking at using CLR, Oxalic, citric or Ascetic Acids, fearing that I would do serious damage to them. So, any advice would be greatly appreciated. 
Cheer
Malachi. 

[Gemster]

G-day Malachi ,   You could try a small area on the 'rocks' with some Citric acid. Start with a weak solution.. Put a few drops on a small area and watch for a reaction... If it does react, quickly place in a bucket of water to diffuse the reaction...  or Try 'Brasso' and rub this on it... Sounds silly but it is a wonder cleaner simular to WD40... 1001 uses... Gumption may also work too....

  Gemster....

[wallie]

Do not use acid on prehnite

[MakkyBrown]

Could get one of these, just don't spray your fingers 
https://www.ebay.com.au/itm/40W-Electric-Cleaning-Spray-Gun-Textile-Spot-Cleaner-Pressure-Nozzle-Set-220V/153196047101?epid=7024229264&hash=item23ab322afd:g:r9sAAOSw7fBbq74J:rk:1:pf:1&frcectupt=true&autorefresh=true

[Jimnyjerry]

Presume a hand held steam cleaner with a nozzle would be a bad idea.
 

[MakkyBrown]

Don't know JJ might work, those ebay spot cleaning sprayers are pretty good. You need to use safety googles, they run at fairly high pressure and could probably almost cut your fingers when turned up. We were using ours to clean geodes.

[MALACHI]

Thanks everyone for your replies. I did try lemon juice ( citric acid ) on a less important and noticeable surface. Nothing happened, no reaction or cleansing happened. Though I didn't allow the lemon juice to remain on the stones for too long; 10 minutes at the most.  I was thinking of trying one of those small domestic steam cleaners made by Karcher.  I was going to test the effect on some other stones first to see how harsh the steamer is, then then make a call based on the observation. 
cheers
Malachi 

[Gemster]

Sounds like a plan Malachi... Let us know what works as others will benefit im sure... You can also buy Citric Acid from the Spices/Cooking section at your local Supermarket and make to your own strength... I use it for cleaning my Pyrite Specimens....

  Gemster....

[grumpy]

hi malachi if you have a compressor a engine degreaser gun from super cheap i use the metal one with a screw top use compressor regulater to adjust pressure as needed watch the fingers with pressure or wear gloves i also have a 3lb tumbler barrell fill as usual ad 1 to 2 desert spoons of  lux flakes leave 1 to 2 days clean stones

[FlashGP]

Hi Malachi,

Minerals.net has the following information. Which may be useful.

"Prehnite sometimes has a brown ferrouscoating, which can be removed with chemicals. Some dealers treat specimens by oiling them with lubricants to enhance luster. Prehnite is named after Colonel Hendrik Von Prehn (1733–1785), a Dutch commander of the military forces of South Africa who brought Prehnite specimens from South Africa to Europe."

Its a silicate with the following chemical composition.
Ca2Al2Si3O12(OH)

Color
Light green, dark green, mint-green, yellowish-green, brownish-green, and bluish-green. Rarely brown, yellow, orange, pink, gray, white, or colorless.

Crystal System
Orthorhombic

Properties
Streak 
WhiteHardness 
6 - 6.5Transparency 
Transparent to translucentSpecific Gravity 
2.8 - 3.0

It may be worth testing CLR on a small bit to dee if it won't react.

Regards
Flash

[darttrev68]

Hi Malachi, the chemical you need is Sodium dithionite. It will remove the iron staining and is not acidic. It also works in cold water and you will see results within an hour. Getting your hands on it is another issue. Some chemical companies may sell it to you, while others require a purchase permit. You could try your local High School science lab or contact the geology dept of one of the Universities.

[MALACHI]

Trevor thanks for your feedback, It would be nice to try something else. So far my attempts on the wavelite and prehnite have failed to totally remove the staining. There a chemical company called Redox west of Melbourne. I hope they will be able to help.