Soda Bar 19 by Bob Keller
Winner 2009 OPLC Faceters Hobnob Most Beautiful Stone Competition
Synthetic Division

Soda Bar 19
Designed by Bob Keller October 2008
An exhibition stone designed by Bob Keller October 2008
Winner 2009 OPLC Hobnob "Most Beautiful Stone" Competition - Synthetic Division
Angles for R.I. = 1.450 17 + 2 girdles = 19 facets
2-fold, mirror-image symmetry 96 index
L/W = 1.613   T/W = 1.000   U/W = 0.179   H/W = 0.770
Vol./W^3 = 0.568 Brightness at 0 degrees tilt for RI = 1.45
COS = 32.2 ISO = 36.3

C1 0.00° Table  
C2 10.00° 96-48 Cut to make face width C1 = .179W
C3 20.00° 96-48 Cut to make face width C2 = .182W
C4 30.00° 96-48 Cut to make face width C3 = .191W
C5 40.00° 96-48 Cut to make face width C4 = .207W
C6 90.00° 96-48 Cut to length L make face width C5 = .234W
P1 68.00° 24-72 Cut to width W (W = L / 1.613)
P2 54.00° 24-72 Meet P1.C5.C6
P3 44.00° 24-72 Cut to make face width P2 = .305W
P4 68.00° 96-48 Cut to desired face width C6 (girdle)

Download PDF format printer friendly cutting instructions: [ Download Icon Soda Bar 19 Printer Icon ]

Download the GemCad format design file: [ Download Icon soda_bar_19.gem ]

I designed and cut Soda Bar 19 as a large exhibition stone and my 2009 entry for synthetic division of the "Most Beautiful Stone" competition held during the annual Tucson Show Faceters Hobnob, hosted by Tucson's Old Pueblo Lapidary Club. Soda Bar 19 is a step cut, "opposed bar" type design, employing bands of parallel bars arched across the crown oriented at right angles relative to parallel bars on the pavilion. This opposed bar arrangement produces a highly dynamic checkerboard pattern that shifts and dances with slight changes in perspective as the stone is viewed. While this design utilizes only 19 facets, the overall effect belies the relative simplicity of the cut and provides a paradigm example of a sum exceeding its parts.

The barrel shape of Soda Bar is generated by the intersection of the arched crown facets with the angled plane of the P1 facets. This geometry is achieved by foregoing a conventional girdle all around the perimeter of the stone. Because I designed and cut Soda Bar as an exhibition stone I was unconcerned with mounting it in a jewelry setting. The C6 "girdle" facets may be considered optional on exhibition sized stones, but I incorporated them in the design to facilitate channel or tension mounting of jewelry sized versions.

Cutting the Prototype Stone

Saw trimming the rough is essential on large stones to save time and wear and tear on your lapsThe blocked out stone is shown here dopped on the pre-existing glass gemstone pavilion and ready to commence re-cutting.

The prototype Soda Bar 19 was re-cut from an 80mm SRB pattern gemstone molded from ordinary soda-lime glass. These inexpensive molded glass gemstones are manufactured in China and are available in 40mm through 120mm sizes in various colors through the Cutting Rocks web site. After some initial trimming down and preforming of the glass gemstone on my 6" trim saw, I decided to dop the existing pavilion in a 1" cone dop and cut the crown side first.

I suggest blocking out the stone width (W) first and then the length (L), which should be made equal to W x 1.613. Verify that your blocked out stone has sufficient height (H), which should be at least a fat tad over W x .770 to ensure you will have adequate material remaining to complete the stone after transfer. More is better here for some insurance in the event you encounter problems requiring recutting like I did...

Progress cutting the crown bar facets is shown at the 600 mesh stage. Bigger Stones = Bigger Fun but can also involve bigger problems. My target size for the prototype Soda Bar 19 was 60mm in length, but several complications during its cutting resulted in a 55mm x 34mm finished stone weighing 250 carats.

The first complication was caused by being lazy and using the pre-existing table of the glass gemstone as a "prepolished" surface for the C1 facet. After cutting and prepolishing the rest of the crown side facets, I discovered the surface of the pre-existing table was nowhere near flat enough to polish. (After all, the "rough" was a molded gemstone... D'oh!) By the time I got C1 properly flattened it had become overly wide, so it was back to recut the entire crown.

I had nearly finished the second round of pre-polishing when the lap grabbed the stone and exerted so much torque on it as to cause the shank of the dop on the pavilion side to fracture nearly clear through and bend approximately 15 degrees! I was lucky that the stone was not yanked completely from my hand and flung and smashed against the side of the cast metal splash pan on my faceting machine.

This was the first and only dop I've managed to break while cutting a stone. It was one of a pair of defectively manufactured 1" Graves brass cone dops ordered late in the game to work on my Spinning Wheel 40 entry for the 2008 Faceter's Hobnob. Upon their arrival it was immediately apparent the cone recesses were not cut clean to the apex, leaving culet-crushing nubbins of brass where the apex should have extended through.

I was under the gun to finish Spinning Wheel 40 and of course all my other large cone dops were currently occupied. So rather than burn precious time waiting on replacements from Graves, I elected to drill out the nubbin myself.

After drilling one out using a drill press, a close inspection revealed the dop was not machined from a single billet of brass. The 1/4" diameter shank is press fit into the dop body, and because I had drilled it out on the deep side to ensure it would accommodate glass angles with deeper than normal culets, I became concerned that I might have removed enough material to compromise the press fit. So I heated the dop with a propane torch and soldered the joint between the shank and body with 50-50 tin/lead solder for additional security. In the process of doing that I may have caused the brass to harden and crystallize, so I can't cast blame elsewhere for the failure.

There were no issues with that soldered dop for a year, but its untimely demise reduced my work on the Soda Bar prototype to a nice preform for the crown. The stone shrank a bit after I redopped the pavilion on another 1" brass dop and recut the crown. While on my second pass at pre-polishing the crown I set the stone down to the lap with a bit less attention than normally exercised and somehow managed to cleave a large, conchoidal chunk out of the edge of the crown in the vicinity of the C3 and C4 facets on one side of it. This mishap necessitated further reducing the size of the stone and yet another recut of the crown so as to regain correct proportions at 55mm x 34mm.

Soda Bar is designed and the facet face width ratios calculated so that when viewed square on to the crown planview, the opposed bars on the crown and pavilion will form equal squares. The edge-to-edge face widths of the step cut facets are provided in the course-by-course diagram instructions as a percentage of the stone width (W) to facilitate cutting the design to differing dimensions as dictated by your inclinations and rough.

As an example, for a stone 20mm wide (W), the length (L) should be 20mm x 1.613 = 32.26mm. The edge-to-edge face width of the C1 facet should be 20mm x .179 = 3.58mm, the edge-to-edge face width of the C2 facet should be 20mm x .182 = 3.64mm, the edge-to-edge face width of the C3 facet should be 20mm x .191 = 3.82mm, and so forth.

A pair of calipers resolute to at least .1mm is needed to make the face measurements. Deviations from the design ratios will produce rectangles instead of squares. However, don't sweat the small stuff, as minor deviations here and there are unlikely to be noticed in the finished stone.

Due to the arch shaped contour of the crown, none of my standard shaped dops fit it properly. After the incident with the broken dop I was becoming paranoid about the greater than normal forces at play and concerned about losing the stone off the crown side dop while cutting and polishing the pavilion after transfer.

I considered filling a standard cone dop with viscous epoxy and allowing that to flow out and fillet in the gaps between the dop and the crown. But being wax dopper at heart, I finally elected to file the crown contour into the rim of another 1" brass cone dop and put my faith in a close fit and black wax. Thankfully this approach worked out and cutting the pavilion side of the stone progressed without any further catastrophic incidents.

While there are only 8 facets on the pavilion of Soda Bar, cutting that side is not without some challenges. The angles formed by the intersection of the P1 and C1 - C5 facets are particularly acute and prone to edge chipping, particularly so with glass. The long keel between the P3 facets is also prone to edge chipping. Conventional advice to help alleviate edge chipping on keels and other acute angles is to orient the stone so these edges are cut facing into the rotating lap rather than trailing them with the rotation. However, that "edge in" orientation is easier to prescribe than it is to accomplish with large stones on conventional 6" or 8" faceting machines.

If you encounter issues with edge chipping, use finer laps earlier in your cutting sequence, dial down the lap speed and increase your patience. My cutting sequence with large glass stones is typically 50/60 for blocking out, then 100, 240, 600, and 1200. I employed 3000 LapidaryPro friable diamond powder and WD-40 on a Batt Lap for the prepolish on this stone. Cerium oxide is my polish of choice for glass and I polished Soda Bar using Spectra Laps on a master lap.

Stainless Steel Dops After finishing the Soda Bar prototype, I have since acquired a set of large cone and flat dops turned in one piece from billets of stainless steel. These were custom fabricated for me by Billy Bob Riley after I related the story of breaking the brass Graves dop while working on Soda Bar. Billy Bob is an Old Pueblo Lapidary Club member and the machinist behind the infamous Monster Faceting Machine,

While not quite as heavy duty as the Monster Faceting Machine dops, these jewels should at least serve to circumvent future dop breaking issues while I'm cutting larger stones on my humble 8" Graves Mark I machine. I don't like to ponder the potential consequences, but I suppose now my machine's faceting head is the weak link in the chain...

Thank-you Billy Bob!

The Results

I once heard faceting described as a never-ending process of learning to resolve and overcome problems. While some learning curve was involved in cutting this stone, I was able to overcome the encountered problems through the application of perseverance and the time honored technique of re-cutting through them.

I think the most significant lesson of the Soda Bar 19 prototype is cutting stones of this ilk involves time and patience. Unlike several previous projects, I began cutting this stone in October 2008, well in advance of its first showing and the Most Beautiful Stone event at the 2009 Faceters Hobnob. Thus I was able to work on it at a casual pace and to think about and deal with the encountered problems in a relaxed frame of mind and without feeling overly pressed for time.

Bragging Rights I actually managed to finish the prototype nearly a month ahead of hard deadlines for attending the 2009 Tucson Show and Faceters Hobnob, an accomplishment that put a smile on my face in and of itself. Needless to say, I am pleased with the Soda Bar 19 results. My prototype stone was voted "Most Beautiful Stone" in the synthetic division by the attendees at the 2009 Tucson Show OPLC Hobnob.

Performance wise, this is one of the "darkest" stones I have designed and cut in terms of its GemRay ray traces, returning COS and IS0 brightness of 32.2 and 36.3 respectively. All said and done, this stone serves to demonstrate there is considerably more going on than brightness when it comes to the perception of beauty in gemstones. It is also cut from garden variety glass, which is a weak material in terms of refractive index and dispersion compared to other popular synthetics, particularly so the CZs that have tended to dominate the synthetic division of the Hobnob's Most Beautiful Stone competitions.

However, the dynamic play of pattern and scintillation within this stone due to the interaction of the opposed bars on its crown and pavilion with the viewer's perspective is truly attention captivating and the primary attractor in terms of its visual interest. My prototype is also a big stone, and as any optical designer appreciates, when it comes to returning photons, there is no substitute for aperture...

I would recommend Soda Bar 19 for intermediate to advanced faceters. Less experienced faceters who are comfortable with step cutting and who've successfully cut and polished a larger stone or two may also find it suitable. I didn't keep track of the total time invested in cutting the prototype stone. While faceting, for me, the journey is the destination... But taking what I learned from cutting the prototype and barring any time consuming complications like broken dops or cleaved edges, I guesstimate it would take me at least 40 hours to cut another in the 50-60mm size range.

I appreciate the ooo's and aah's my Soda Bar 19 fetches when shown off, and it has served to impress me with the potential of opposed bar type cuts. I plan future work on several variations of this design including an even larger exhibition version incorporating additional bars, and several jewelry sized versions optimized for higher refractive index materials.

My prototype stone displays on a simple, custom designed and fitted stand that I fashioned from some scrap dowel and basswood sheet, an accouterment I consider essential for the proper display of large stones. If you cut a big Soda Bar 19, or some other large stone for exhibition and general showing off, do not underestimate the effect and importance of a stand upon its presentation.

If you should cut a Soda Bar 19 of your own, enjoy, and let me know your results.

On a closing note, I'll mention I'm planning to have some more fun designing and faceting large recuts from those molded glass SRBs. If you have an interest in these, check out the Cutting Rocks Web Site to see what is available.

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Bob Keller