Plume Agate Doublet - How It Was Done

[bobby1]

How it began. I initially used a long  oval template and roughed out the cab from this piece.

The preform back had lots of vugs, pits and such that I had to remove to make the cab but in removing them the cab preform ended up rather thin to cab and still expect it to be durable as a large cab. I decided to back it with a slab of Basenite which would make it durable as well as enhancing the contrast of the plumes. Here it is with the Basenite glued to the back. I used a plate of glass and tumbling grit to grind both pieces perfectly flat and then mixed up and applied Epoxy 330 to glue them together. I let it set overnight to allow the adhesive to cure.

Side view.

I ground the Basenite backing down to the pre-shaped preform's girdle.



Here I did my first grind in the process of creating the dome. I ground it at a 45 degree angle down to within about a 1/16" from the base of the Agate. I wanted to have a slight straight side on the Agate.
Side view.

Top view.

Here is the next pass around the preform. I went in about 1/2" on this pass.
Top view.

Oblique view showing the area on top that remains flat.

Another view.

This next step is where I start to remove the flat top. Here I go from grinding to a 60  grit slightly used SiC sanding belt. I will use it to take all the needed material off to complete the dome. Because the preform started out rather thin I have to work carefully to remove the flat but still have a complete dome.
Here it is almost fully shaped.

Another view of the finished coarse sanding.

Now I start the work on the girdle. I sanded it perpendicular to the top and bottom of the cab.so that the small edge of the Agate will have a finished surface to polish.

Here it is sanded to a 220 grit surface.

Next I polished the front of the cab.

Because I didn't want any of the black from the Basenite to show when the cab is viewed from the front I ground a slight reverse taper to the Basenite, and sanded it.
 
Another view.

Next I ground a very slight 45 degree bevel to the back edge of the Basanite layer and sanded it. The purpose of this bevel is to remove any chips that had occurred during grinding as well as providing a safer edge if the cab were to be set.
 
Next I sanded the back in preparation for polishing.
 
I rechecked the polish on the front.

I polished the girdle of the Agate and Basenite.

I polished the small bevel.

I polished the back.

The finished cab.

Bob

[NuevoMundo]

Quite a bit of work, but a stunning cab! The plumes look much better than they ever would on  a 1/4" slab... Thanks for taking us through your process!

[hulagrub]

Awesome!

[Enchantra]

Bobby...
I love seeing this step by step version of how you do your amazingly huge cabs. 

Those plumes just jump at you, the basenite was a great choice for backing. 

[Ajo]

EXCELLENT! Your precision is incredible. I've always liked you cabs. Nice job! Eric (Ajo)

[lithicbeads]

 That was fun to read and see. I hope people can at least begin to appreciate your expertise and your careful craftsmanship.

[Helene]

Thanks for the in-depth tutorial.  Your right about the backing making the cab stronger.  I noticed that Basenite is often used as a backing and it does look good, but often wondered the difference in visual appearance using a white stone backing.  With the transparency of the plume, would the cab look more natural, or does the black backing give your cab an added visual pop?  

Another thanks Bob for sharing your process with us.  

[lonelygems]

Great tutorial.....this is usefull for a clear and nice material, thank you for sharing Bob....

[3rdRockFromTheFun]

Pretty darn awesome Bob - thanks so much for sharing that in thorough detail - things like this help us all a great deal!

[legendarygranite]

Awesome! Love the play by play. I noticed on the last picture, which looks like you took the pic directly over the top of the cab, I couldn't see the bottom of the cab, it was  my understanding that the bottom of the cab should be tapered out from the start of the dome, so the cab won't fall out of the setting?

[bobby1]

Jason,
I usually do an inward taper on my cabs so that if or when they might get set the metal will be formed over the taper to hold the stone in place. This one is an exception because of the black backing. When I'm doing an Opal doublet or triplet it will also have this reverse taper to prevent seeing the black backing. To set these stones the bezel will have to be made a little taller so the top edge will form over the top edge of the stone. You have to be a  little more skillful to set these types of stones but it isn't that hard to do. Even though I doubt that a cab of this size or larger will ever be set I still do the reverse taper to keep the black from showing especially if I'm going to be displaying them in my case. On all other cabs that I make regardless of size I do the slight inward taper as if it were ever to be set. I can't imagine who or for what reason someone would want to set one of my really large (4" to 6"across) cabs.
Helene,
I use black backings, clear quartz backings and translucent backings depending on what effect I'm trying to achieve. In this case I wanted to see what it would look like to have a slightly darker background to enhance the plumes. I'm happy with the outcome.
Last week when I was at the CFMS show I got into some in-depth discussions with some of the judges (and judge trainers) about displaying various things. Two years ago I entered a successful case on cabs. One of the things that the rules state is that the largest dimension that a cab can have is 100mm. This excludes most of my cabs from competition in the cabs category. I asked Dee Holland (the guru on judging who is instrumental in writing the rules) why there was this limit and he said it is because in the past some people had ungodly looking huge 1' across "cabs" that they claimed were "wearable" as jewelry so the rules were rewritten to put a limit to such nonsense.
I have always wondered if I could enter my case in a category that was allowed. My case has cabs, jewelry, metal work and carving, a mixed bag of stuff. To my surprise after they looked at the case they said yes! There is a category (I forget which one -  I'll have to read up on the rules to see) and it could be entered as is. I going to do it at the next CFMS show and see what happens. They haven't  found a club to sponsor the 2014 show yet but there are some good prospects, though.   
Bob

[SEW1950]

"My case has cabs, jewelry, metal work and carving, a mixed bag of stuff."

    Wanna share?

[christopherl1234]

I admire your lapidary skills Bob and enjoy seeing your large cabs. You seem to capture some really capture some really interesting patterns that would otherwise be lost in a smaller cab.
Thank you for sharing your technique.

[Helene]

Thanks for your explanation on choosing backing color.  Helpful you are.  Good luck in your competition entry.  let us know.

[bobby1]

Here is what I usually display at the shows. It changes somewhat because I like to display different cabs each time I show it.
Bob