KOH (Potassium Hydroxide) on Texas Echinoids

[JamieLynn]

With the encouragement of @JohnJ I have started experimenting with KOH - Potassium Hydroxide! I was quite unwilling to try - I am not much for prep work...don't like the tedium. I'd rather just find a better fossil! BUT...sometimes you find something that needs some revealing -and finding another better one is not really an option! SO.....with a push from John and some good "how to" instructions I decided to give it a try, with great trepidation! 

 

I started with three Phymosomas and a Leptosalenia (Texas Cretaceous). John said to soak them for an hour to keep the KOH from penetrating into the inside and potentially causing damage.  After setting them up in a well ventilated area, I applied the KOH flakes on the matrix I was trying to break down.  I started with a 30 minute sit and then rinsed and toothbrushed with water and vinegar (to neutralize the base KOH).  Then soaked for 15 minutes.  I took photos after each session to show the progress. 

 

The initial 30 minutes didn't seem to make much of a difference, so I did the second session at one hour and the third session at 1.5 hours. I did one last session at 2 hours and here is my takeaway:  It seems that the difference in amount of matrix removal from 1 hour to 2 hours was not really that much, so 1 hour seems to be the best option - which means I could do multiple sessions in a day. 

 

Here are the results on the Phymos and the Lepto.  I then did three other fossils, one which turned out BEAUTIFULLY and the others need a little more work to get them finished up. 

Leptosalenia texana:

Before and After:

Before,  1 Hour,  1.5 hours and 2 hours

 

Phymo 1 Before and After

Before, 30 minutes, 1 hour, 1.5 hours

Phymo 2 

Before and After

 

The Third Phymo had the most matrix on it and it really didn't do a heck of a lot, so it is best to get rid of as much matrix as possible before. Next time I will scrape down as much as I can before starting the KOH. 

 

SUPER happy about how this one turned out: It was a nice fossil to begin with but had a "sheen" of matrix that was obscuring the nice details. KOH did an amazing job on it: Before and After

One hour sessions: 

 

Final one (a tetragramma!)  still needs some work. I will do some more sessions on it: 

I am being careful to not get the KOH on the exposed matrix (where the test is gone on the bottom quarter) 

 

 

[DPS Ammonite]

Can the KOH be neutralized by other acids such as HCl? How do you know that the KOH is fully neutralized? Too much acid will start to dissolve your calcite fossils.
 

 

PS: let me answer my own question: you’ve added too much acid if a piece of calcite starts to bubble.

 

 @oilshale

Edited Tuesday at 08:11 PM by DPS Ammonite

[JamieLynn]

I just dip the toothbrush in vinegar and give it a quick brush and then dip in water. Repeat repeat and then soak in water for an extended time. So vinegar is not on there for very long at all. I have tried the diluted vinegar soaks for prep to not good luck, but the quick brush and rinse neuatralizes the KOH.  I do not know about other acids. 

[JamieLynn]

If a chalky residue is still present after the last soak, it means the KOH has not been fully neutralized and you do the vinegar toothbrush again. 

[Mikrogeophagus]

Those are some nice results!

[JohnJ]

Great images, Jamie!  There is definitely an 'art' to the process that is only gained through experience with different matrices and specimens of varying condition.  When you get it right, the results can be amazing.  Good work!

[Coco]

Very good work. It’s very interesting to see the progress at various points of preparation with the time schedules.

 

Coco