What is the green film on my fossil

[jeannie55]

I have noticed four of the fossil rich rocks that I found last Friday have a greenish-black film that is very hard to remove and that comes back after a few hours. I found all four near each other and they were all covered in reddish clay. The main fossils are coral animals, sponges, and layers of oyster shells. The matrix is jasper, crystal, and quartz, with one of the four having a few small gravel-like stones. I suspect the film may be rust if that’s possible because on the work space I also had two pieces of hematite and neither have the film.  If it is rust, would using a baking soda wash help? I am posting a picture but it’s with my phone so I apologize for the lack of quality. Soon I will post better pictures. If I am unable to post the pictures, I am sorry and will try again when my grandkids get home from school. 

[jeannie55]

I could only upload one so hopefully you will see the film. Thanks so much for any information any of you may offer. 

[Fossildude19]

1 hour ago, jeannie55 said:

I could only upload one so hopefully you will see the film. Thanks so much for any information any of you may offer. 

You can always add a photo via the reply button. 

If you get a message that you are out of space, or can't upload anymore, refresh your browser.  

[Fossildude19]

Could be dried algae, or mineral staining, in which case, you won't be able to remove it. 

Air abrasion may work, but I wouldn't count on it. 

[jeannie55]

After I washed the dirt off of it, the film wasn’t present.  It showed up a couple hours later. It sort of reminds me of the green stuff on my grandmothers silver that she left to me only because she knew I hated cleaning the stuff.  I use baking soda on the pieces before I use them. I rarely use them. Thanks for replying. 

[pachy-pleuro-whatnot-odon]

For cleaning silver you can also use a fine chalk powder, such as a pulverised chalk stick used to write on old-fashioned school blackboards. Works wonders without doing overly much damage to the silver work - which is, in the end, what most other methods do: abrade the top surface off, so that the oxidation is removed and the clean, unoxidised layer underneath, remains.

 

As to what the film could be: I have absolutely no idea. Nor can I make out too much from the photograph you posted. If it is organic in origin, you could try cleaning it with a bleach solution (tiny bit of bleach diluted with water [remember Always Add Acid]) and brushing with a toothbrush, rinsing in clear water frequently. However, as the fossils are made out of limestone themselves, I'd be careful applying such a technique, as it may damage the fossils for more than it's worth. Therefore, always first try in an unnoticeable spot. I've had very good results removing green marine algae this way. I doubt yours is the same problem, though, as it'd be odd for algae to first disappear and then reappear like that...