Updated February,22, 2015

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Specimen Preparation - How to Clean Them

By, John Veevaert
Trinity Mineral Co
Weaverville, California

So, what does it take to get a cleaned specimen of benitoite and/or neptunite?  Attractive specimens of this beautiful mineral assemblage just don't happen - they are literally crafted!  The process involves several steps which are all time consuming and require a bit of artistic aptitude.  One thing to keep in mind the whole time is that removing matrix is a one way procedure.  Once it is removed it can not be put back. Too much removal is as detrimental to the finished specimen as leaving too much is.

Here is a list of the equipment I have to get the job done and the chemicals I use:

12 inch tile saw
diamond band saw

Zuber trimmer
water gun
Air compressor
Air Scribe
Hot plate (to melt wax)
Small paint brush to apply wax
Dremmel Tool
Hydrochloric acid
Phosphoric acid
Iron Out
Ammonium Bifluoride (VERY dangerous stuff - I can't over stress this point!!!
Sodium Hydroxide
Wax (etch inhibitor)

Initially, uncleaned specimens are rinsed in water to remove as much of the clay and mud as is possible before immersion in the acid bath.  Mud and clay from this mine is some of the most tenacious crud in the world.  It is really stuck on there!  After the initial rinsing I place the specimens in an acid bath (the strength of the hydrochloric (HCl) acid (muriatic acid is the same thing as HCl) at 5-7 parts water to 1 part acid) to do a preliminary etching - generally removing 1-2 mm of the natrolite.  

I then take the specimen to the saws to trim excess matrix and preform the specimen.  I also use the tile saw to remove one half of a sandwich vein specimen.  You have to make a judgement call and sacrifice one half of the specimen usually.  All too often I end up munching the wrong half. Use your best call. I get it right about 80% of the time.

In this example you can see that I chose to remove the side where I did not see benitoite crystals.
But I could not see through the natrolite and sawed two benitoite crystals in half...Most of the crystals
will be on the side that I chose but sawing through a crystal of benitoite is still a sort of crime against nature.
Regardless, I have a specimen now ready to wax and etch that will produce a nice benitoite specimen when finished.

So here is the specimen after it has been etched.  It took 13 days to complete this specimen and I had no idea 13 days ago
that this thin vein was filled with very lustrous crystals to 8 mm across! In fact I am keeping this specimen for my collection!

Next the specimen is assessed for waxing needs (paraffin or wax are the same thing).  Wax is used to impede etching where natrolite is desired with the finished specimen.   I use a hot plate and melt the wax in an old pot then use a brush to dab it on in places I want protected.  I also frequently dip the sides of the specimen in the melted wax where it is easy to do so which speeds that step up a lot.  

Once the desired waxing is completed I place the specimen in an acid bath with the strength of the hydrochloric (HCl) acid (muriatic acid is the same thing as HCl) at 5-7 parts water to 1 part acid. The natrolite is slowly dissolved by the acid (The natrolite dissolves like salt does in water. There is no effervescence!) which can be removed by running water and/or very light brushing.  If the acid strength is too high you will find that the natrolite dissolves but leaves a jelly (opal actually) residue and then you have a problem! 

As crystals begin to emerge from the natrolite it is necessary to pull them from the acid bath and then rinse them thoroughly, dry and apply more wax to portions of the specimen to prevent an undesired loss of natrolite (The natrolite provides a pleasing contrast to both the benitoite and neptunite crystals. Too much etching leaves some crystals precariously perched by a thread or totally ruins the aesthetics of the specimen) Once the desired amount of wax is applied the specimen is returned to the acid bath and etching continues.  You do this as often as necessary to reach the desired level of etching.

When you are happy with the etching the wax must be removed which is done by a heated water bath.  First I rinse the specimen and soak it in water for 6-12 hours.   Then I place the specimen in a pot filled with a solution of water and sodium hydroxide (base) and place it on a hot plate.  The solution with the specimen is then brought to about 80-85 Co or about 180 Fo. The heated solution melts the wax and the base acts as a wetting agent to lift all of the wax off the specimen and also to completely neutralize the acid that entered the specimen during the long periods of soaking.  

Once the wax has melted off the specimen I let the pot cool slowly to room temperature.  The melted wax floats to the surface of the pot and when cool is easy to scoop out and reuse if you want to.  The specimen is then placed in water and allowed to soak for a 6-12 hours.  I do this 2-3 times. 

Final cleaning can be done with ultrasound bath and an immersion of the specimen in a solution of ammonium bifluoride (folks this is VERY, VERY, VERY, VERY dangerous stuff.  If it comes into contact with your skin you will kiss that skin good bye in a few days and you will be left with a long term physical problem and potentially worse!).   This chemical will really brighten up the neptunite and will aid in removing any excess gel that formed during your etching process if your acid bath was too strong.  I advocate the use of a toothpick or dental tool and a quick bath in an ultrasonic as the final step in lieu of the above chemistry.

To be done right an average specimen requires no less than 3 days of personal attention.  Many require a great deal more!  In the past I have personally spent over 2 weeks on one rock!  Doing the work with batches of specimens is the only way.  It is easy to see why specimen prices are what they are for this rare mineral given the amount of time it takes to make these specimens look the way they do!  Trust me, the labor costs of cleaning these specimens correctly is never fully recovered...

A final word here on benitoite from the Benitoite Gem Mine.  I am guessing that 98% of all benitoite crystals have a matted luster.  The pinacoid and prism faces will be lustrous 9 times out of 9. The pyramidal faces, however, will not.  That is just the way Mother Nature created these things.  Some crystals will be lustrous on all faces and those specimens are rare.  The other thing to remember is when something has sat around for millions of years some fracturing is likely to occur. The vast majority benitoite crystals were fractured though eons of natural processes. When you are finished etching the specimens with benitoite you will likely see the fractures as the specimen dries.  There is nothing you can do to change this fact, so accept it. I etch hundreds of specimens a year and find the severe fracturing probably 80% of the time and put most of those specimens in the wholesale flats.  That is why the high quality benitoite specimens command high prices.  They are, unfortunately, the exception to the rule...

The following are a set of pictures to show what specimens looked like before they were cleaned and after.  They also show several of the steps needed to take what looks initially like a piece of rubble and transform it into a finished specimen of high quality.

I have selected a specimen that initially weighed 590 grams (0.59 kilograms).  It's initial dimensions were 12x 11.8 x 5 cm.  Final dimensions are 12 x 9 x 4.2 cm and it weighs 430 grams (0.43 kilograms).

This shot shows the specimen after its first 1 hour soak in acid. Note the large  chunk of crossite before it was removed.  There are several crystals of neptunite visible that are all damaged except for one.  Also visible are crystals of benitoite and joaquinite.  In my mind this has the potential to be a very good joaquinite specimen. Here's the rock photographed from the other side.  At this point I have already waxed it along the edge and I have covered the undamaged crystal of neptunite. 


These two shots show the wax that has been applied to the edge of the specimen.  Note that I have not entirely covered the benitoite crystal visible.  I wanted to make sure that it was not completely bared of all natrolite until I was sure that the joaquinite was a plentiful as I thought it would be.  This shot shows waxing along the edge and also a number of joaquinite crystals and a busted neptunite.


Here's the before right after the good neptunite has been waxed.  Note - I covered the entire area here and later had to remove the wax and then re-wax the specimen.  This shot shows the same area after etching and cleaning are completed.  There is absolutely no sign of the additional neptunite crystals next to the one that was protected.  Also note that the damaged crystal next to the good one is gone.


Here's the rock after several days of drying after the last rinsing.  Note that there is a yellowish staining.  This is evidence of acid that leached to the surface.  Still not done with this thing!! Fortunately, water soaking and rinsing are all that are needed to cure the problem.  All that is needed is time.  In this case about 24 hours. 


Here's a closer view of the yellow staining.
And after the final rinsing.


Here's a close view of a residual damaged neptunite crystal. 
And here is the same view after the damaged crystal has been removed using a dremmel tool.  There is a little sacrifice of some small joaquinite crystals but it is acceptable given that there are literally hundreds of joaquinite crystals on this specimen.


Here's a close up of a 1.0 cm crystal of benitoite in a field of 1/2 cm crystals of neptunite and dozens of 1 mm crystals of joaquinite.  Here are two damaged crystals of neptunite that no longer exist in the final specimen. 


Here's the final deal.  Hundreds of joaquinite crystals as I suspected and two 1+ cm crystals of benitoite and dozens of additional undamaged neptunite crystals to 1.9 cm in length that were completely hidden!!  I have left scattered areas of natrolite which provide a nice contrast but removed enough to show the large number of joaquinite crystals.  There are more under the natrolite  but enough are exposed to get the point across.  Also note the missing chunk of matrix visible in the photo on the right.  That was removed using a Zuber Trimmer.
And here it was at the very beginning. 

Total time from start to finish about 8 days.
Actual time I spent on this specimen about 12 hours.
This specimen sold for $500.00

This specimen would have cost someone about $39.00 if it had been placed in an order of mine run.

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