Tue, 12 Sep 2006

Visible Effects of Painting & Digging on Superideal Diamonds

With
grateful acknowledgment to John Pollard, Sergey Sivovolenko, AGSL and
GIA Laboratories for valuable resources and input. Graphics produced
with DiamCalc software by OctoNus.

Note: This
article pertains only to ‘superideal’ diamonds, meaning those with
commonly agreed ‘ideal’ proportions and a high level of optical
symmetry. Our understanding is that this diamond category makes up less
than 1% of all round brilliant diamonds produced. The effects can be
quite different on deeper and shallower diamonds.

Preface: Common Questions

Q: What are painting & digging?

A: They are approaches taken in final stages of polish, or brillianteering.

Painting shifts
upper/lower girdle facets toward the mains so angles are closer
together. The result is that average girdle thickness where the half
junctions meet is greater than average thickness where the mains meet
(diagram below).

Digging
tilts upper/lower girdle facets away from mains so angles are farther
apart. The result is that average girdle thickness where the half
junctions meet is thinner than average thickness where the mains meet
(diagram below).

For a more technical description refer to this GIA publication: http://www.diamondcut.gia.edu/pdf/6_05_RDR_pg239_243pdf.pdf

Q: Why Paint or Dig?

  1. To
    try and retain weight at critical points: If a diamond is close to a
    commercially important weight the painting approach may be used because
    less material is polished away. On the example below normal indexing
    would result in a finished diamond weighing 0.99 ct. Painting to 4
    degrees – particularly on the pavilion’s long lower girdle facets –
    allows the commercially important 1.00 ct mark to be ‘saved.’

  2. To
    improve a verbal description of girdle thickness or clarity: For
    example, a very thick girdle may be dug out enough so that it will
    appear only ‘slightly thick’ to the grader, or an inclusion could be
    dug-out in order to improve the clarity grade.
  3. Relative
    to crown-only painting: To acquire desirable visual properties. A
    measure of crown-only painting on ‘superideal’ diamonds can improve the
    diamond’s brightness and increase the amount of visible broadfire
    dispersion.

Q: Why is it said that Painting & Digging are bad?

A: Because
the most common uses of painting & digging are ‘swindling
strategies’ in answers 1 and 2, above. What is not commonly known is
that a measure of crown-only painting on superideal makes can be used
to acquire desirable visual properties. Remember that the ‘superideal’
category of diamonds makes up less than 1% of all round diamonds
produced.

The purpose of this article is to illustrate the
differences between crown-only painting (which can be positive) and
pavilion painting or digging of any kind (which are negative).

 

Painting and Digging Performance Comparisons

Painting & Digging Categories

 

Painting
and Digging can be divided into several categories. For purposes of
this article we will discuss the following three primary combinations
(others follow in our appendices).

1A. Crown-Only Painting
1B. Crown-Only Digging
2A. Pavilion-Only Painting
2B. Pavilion-Only Digging
3A. Crown & Pavilion Painting
3B. Crown & Pavilion Digging

Measuring Painting & Digging

Painting
and digging will be measured in terms of degrees of average deviation
from azimuth, or normal indexing. On a standard round brilliant 11.25
degrees (or less depending on configuration) is maximum.

ACP = Average Crown Painting. APP = Average Pavilion Painting

ACD = Average Crown Digging. APD = Average Pavilion Digging

Note:
Painting & Digging can also be measured in ‘clicks’ which
correspond to a diamond cutter’s tang. 1 click is equivalent to 3.75
degrees. 3 clicks, or 11.25 degrees, is maximum.

ASET Comparison Chart (generated using DiamCalc software by OctoNus)

We
believe this charts supports the considerable difference we have
observed between crown-only painting (1A) and all other painting &
digging categories.

See this link for an explanation of ASET.

1A. Crown-Only Painting:
No reduction in performance under the table. Gradual impact occurs
along the girdle. In our experience the effects are not visible until
ACP of 6-7° degrees (increased greens). The extent to which they are
visible depends on overall configuration & optical symmetry. *Note
that this applies to “superideal” diamonds with no pavilion painting or
digging.

1B. Crown-Only Digging: In our experience
reduction in performance begins with ACD >= 3°. Girdle facets fall
off in performance especially fast (severe greens).

2A. Pavilion-Only Painting:
Visible effects are notable with APP >= 3° (increased greens) and
significant darkness under the table appears with APP >= 7° (severe
blues).

2B. Pavilion-Only Digging: Reduction in
performance under the table with APD >= 2° (severe blues). Girdle
facets fall off in performance especially fast (severe greens).

3A. Crown & Pavilion Painting:
In our experience visible effects are notable with ACP+APP >= 4°
(increased greens). Significant darkness under the table appears with
>= 7° (severe blues).

3B. Crown & Pavilion Digging:
Reduction in performance under the table with ACD+APD >= 2° (severe
blues). Girdle facets fall off especially fast (severe greens).

 

Other combination charts appear in Appendices.

 

Crown-Only Painting – Actual Photos
These 6 subject diamonds have crown-only painting. They range in ACP from 0.5–6.7 degrees.

Their pavilions have negligible deviations; all less than 0.5 degrees.

All 6 have premium ‘ideal,’ near-Tolkowsky proportions.

Ranges: D 60.4-61.6, T 54.1-56.9, C 34.6-34.9, P 40.6-40.9, S 50-55, LH 75-78

All
6 display high levels of optical symmetry or ‘Hearts and Arrows,’
assessed as ‘true’ or ‘near-true’ by standards published at this link.

All
6 range in size from 0.71 – 0.76 ct. Their color and clarity range from
E-G and SI1-SI2 (with the exception of subject 4 which is I VVS1) on
grading documents from the AGS and GIA.

Note: Because of the
difficulties inherent in photographing fine distinctions in appearance
aspects, the images shown in Photos 1 and 2 may not perfectly match
what is seen when the actual diamonds are viewed face up.

Photo 1: Fluorescent lighting, white background. (L-R: ACP 0.5°, 1.5°, 3.3°, 4.0°, 6.2°, 6.8°)

Photo 2: Fluorescent lighting, black background. (L-R: ACP 0.5°, 1.5°, 3.3°, 4.0°, 6.2°, 6.8°)

Photo Compilation 3: Corresponding Ideal-Scope images

Photo Compilation 4: Corresponding ASET images

 

 

Crown-Only Painting – Assessment (AGS, GIA, OctoNus)

The AGS
performs ray-tracing analysis on each diamond it cut-grades. A diamond
of any configuration must return specific numeric values for
brightness, dispersion, leakage and contrast to earn the top grade.
Painting & digging judgments are incorporated into the overall
face-up performance assessment.

The GIA scans each
diamond and uses the reported proportions to determine cut grade.
Metrics obtained from the girdle scan are used to establish a painting
& digging value. The same brillianteering judgments are applied to
all configurations.

The OctoNus Helium scanner is an
extremely accurate scanning device. OctoNus has developed a chart with
verbal descriptions for degrees of painting or digging, found at this link.

Here
is how the 6 subject diamonds would be assessed in these systems
(OctoNus verbal descriptors are being adjusted at the time of
publication – this article will be edited to reflect those changes):

Diamond #1. ACP 0.5 degrees: AGS 0 Ideal, GIA EX, OctoNus Negligible
Diamond #2. ACP 1.5 degrees: AGS 0 Ideal, GIA EX, OctoNus Small
Diamond #3, ACP 3.3 degrees: AGS 0 Ideal, GIA EX, OctoNus Moderate
Diamond #4, ACP 4.0 degrees: AGS 0 Ideal, GIA EX, OctoNus Moderate
Diamond #5, ACP 6.2 degrees: AGS 0 Ideal, GIA VG, OctoNus Large
Diamond #6, ACP 6.8 degrees: AGS 1, GIA VG, OctoNus Large

With these configurations:

Beyond 7.0 degrees would be AGS 1-2, GIA G. OctoNus describes as Very Large
Beyond 8.2 degrees would be AGS 2, GIA G, OctoNus describes as Maximum

Notes

  • Subject diamonds #1, #2, #3 and #4 receive the top cut grade from both AGS and GIA.
  • Subject
    diamond #5 earns numeric values for 0 ‘Ideal’ light performance when
    ray-traced by AGS. GIA reduces it by one grade based on the girdle scan.
  • Subject
    diamond #6 earns numeric values for 1 in light performance when
    ray-traced by AGS. GIA reduces it by one grade based on the girdle
    scan.

Summary
Even
moderate pavilion painting (2A, 3A) and digging anywhere on the diamond
(1B, 2B, 2C) should be downgraded. However, crown-only painting (1A)
should be treated differently, as it does not reduce performance until
extremes and can be used to positively enhance optical properties. This
agrees with the American Gem Society’s diamond-specific ray-tracing
research, studies at Moscow State University and independent leaders in
the field of optics.

To this end, we believe crown-only painting should be independent of other painting/digging judgments.

I
hope this article allows others to understand that the stereotyping of
‘painting’ or ‘digging’ is erroneous when the diamond is not in-hand.
To make decisive evaluations one must know the diamond’s configuration,
what type of painting or digging was done, and to what degree. To
speculate or stereotype will lead to invalid conclusions.

This
study reinforces my assertion that the pavilion of the diamond is of
prime importance. I have always maintained that “it’s all in the
hearts:” Precise, symmetrical pavilion construction at normal indexing
is the foundation for premium performance and has been since long
before we had wonderful tools like DiamCalc and ASET which are
scientifically reinforcing my long-standing position, learned over five
generations of experience and research.

 

Appendix A: More Comparison Charts
Look from left to right and top to bottom to make relative comparisons.

Example: Comparison Chart 2. Crown & Pavilion Both Painted

With 0° pavilion painting there is negligible difference as the crown is painted to 6°.

With 1° pavilion painting the re are blues under the table and slight increases in green out to 6°.

With 3° pavilion painting there is less blue and more green at the girdle out to 6°.

With 5° pavilion painting there is even less blue and even more green out to 6°.

 

 

Appendix B: Wire-Frame Girdle Profiles

p align=”center”>These illustrations show relative degrees of painting & digging as seen in the girdle profile.

 

 

Appendix C. Performance Comparisons

0. Normal Indexing : Values nominal


1A. Crown-Only Painting to 6° : Values Nominal


1A. Crown-Only Digging to 6° : Reduction in light return


2A. Pavilion-Only Painting to 6° : Major Reduction in contrast


2B. Pavilion-Only Digging to 6° : Reduction in light return


3A. Crown & Pavilion Painting 6° : Major reduction in contrast


3B. Crown & Pavilion Digging to 5° : Severe reduction in light return


Crown & Pavilion Digging to 11° : Reduction in light return. Severe reduction in contrast.


Crown Painted to 11°, Pavilion Dug to 11° : Major reductions in light return and Contrast.


Crown Dug to 11°, Pavilion Painted to 11° : Major reductions in light return and Contrast.


 

Appendix D. Enlarged ASET Comparisons
1A. Crown-Only Painting:
No reduction in performance occurs under the table. There is gradual
impact along the girdle but this is not visible until 6-7° degrees
occurs (increased greens).

1B. Crown-Only Digging: Reduction in performance begins when digging to 3°. Girdle facets fall off in performance especially fast (severe greens).

2A. Pavilion-Only Painting:
Visible effects are notable once 4° is passed (increased greens) and
significant darkness under the table appears after 7° (severe blues).

2B. Pavilion-Only Digging:
Reduction in performance under the table even at 2° (severe blues).
Girdle facets fall off in performance especially fast (severe greens).

3A. Crown & Pavilion Painting:
The visible effects are notable once 4° is passed (increased greens)
and significant darkness under the table appears after 7° (severe
blues).

3B. Crown & Pavilion Digging: Reduction in performance under the table even at 2° (severe blues). Girdle facets fall off especially fast (severe greens).

 

by Whiteflash.com

Whiteflash.com