Removing yellow staining/iron(hydr)oxides from competent shale

[paleoflor]

L.S.,

 

Last fall I've collected a number of plates with sphenopterid foliage from the Piesberg (see photos below). While some of the specimens occur in high-quality rock (e.g., Specimens A-C), others show extensive yellow coloration due to water permeation and creation of "rust minerals" (mostly limonite-type minerals). In Specimens D and E, in particular, this mineralisation obscures the detailed morphology of the plant fossils. Does anyone have experience with removing yellow stains from shale-type rocks? I've been reading about rust removers for pavement and garden rocks but would like to inform myself as best as possible before starting a few experiments on leftover fragments. My goal would be to remove the stains to the point where I can more clearly see the plant imprints. These plants occur in quite competent shale. I think it can easily survive being wetted, but complete submersion for extended periods of time would probably be a bit much...

 

Thanks for your help!

 

Tim

 

Specimen A - part and counterpart

 

 

Specimens B and C

 

 

Specimen D - part and counterpart

 

 

Specimen E

[connorp]

Can't help, but I just wanted to say that those are great specimens!

[Ptychodus04]

I have no direct experience with this type of fossil but we regularly use Iron Out to remove iron stains from pyritized fossils. Typically, we would soak them in the solution for a day. You could try brushing on a solution of iron out and water and then washing it off after several hours. I don’t know if this would do anything for your specimens as the iron removal process is typically slow.

[paleoflor]

15 hours ago, connorp said:

Can't help, but I just wanted to say that those are great specimens!

Thanks! The penultimate pinna of Specimen E is over 10 inches long and seems to be connected to the "branch" visible in the top right corner. I'd love to get this particular specimen cleaned up a bit.

[Lone Hunter]

Tried hydrogen peroxide?  Personally I think they are so unique, almost three dimensional, colors are gorgeous and really pop out.

[paleoflor]

2 minutes ago, Lone Hunter said:

Tried hydrogen peroxide?  Personally I think they are so unique, almost three dimensional, colors are gorgeous and really pop out.

 

Thanks for the suggestion. No, I haven't tried hydrogen peroxide on Piesberg material yet, but I have done some experiments on fragments of other organics-rich shales and coal balls; on those materials it caused quite significant overall discolouration. I'm currently out of hydrogen peroxide, otherwise I would have done a test to simply see the effect...

 

I agree with you that the yellow colour is not that ugly at all. Problem is that in some of the specimens it obscures details of the fossils I'd like to see better. Specimens A-C will be left untreated anyway. I now started a simple test with "HG roestvlekkenverwijderaar" (something similar to Iron Out, which was suggested by @Ptychodus04) on a leftover fragment that suffers strong staining. Appears I will need a lot of patience, as the product turns purple as soon as it hits the rock surface. Will report back if I find anything that works particularly well. On a positive note, it does not seem to negatively impact the gümbelite that comprises the fossils.

[Lone Hunter]

Ooh purple, now that would be awesome lol   I had some luck using vinegar and bar keepers friend to remove stains, it had some chemical reaction together, a little topical soak of vinegar then soft brushing with BKF and let it sit for awhile. If nothing works I would still not be disappointed those are just stunning.