1. Scope
This document describes the guidelines for density determination
as it relates to forensic glass analysis. The guideline outlines
density measurement using density gradient columns, analytical balance
and plummet, density meter, and sink/float (comparative) methods.
The guideline does not purport to address all of the safety problems,
if any, associated with density determinations. It is the responsibility
of the user of this guideline to establish appropriate safety and
health practices and to determine the applicability of regulatory
limitations prior to analysis.
2. Reference Documents
|
2.1.
|
Scientific Working Group for Materials Analysis
Documents |
|
|
Trace evidence recovery guidelines
Quality assurance guidelines |
|
2.2.
|
American Society for Testing and Materials
Standard |
|
|
C 729 Standard Test Method for Density of Glass
by the Sink-Float Comparator E1492 Practice for Receiving Documenting,
Storing, and Retrieving Evidence in a Forensic Laboratory |
3. Terminology
Density is mass per unit volume (g/cm3).
Density gradient is a column containing a liquid mixture
that undergoes a gradual decrease in density from the bottom level
to the top level.
Density meter is an electronic device for measuring density.
Plummet is a sealed, immersible chamber of known volume and
density.
Temperature controlled column is an item specifically made
to contain density liquid and remain at a constant temperature.
4. Summary of Guideline
The density of an unknown sample of glass is determined by placing
a glass sample into suspension in a liquid solution. The density
of that liquid is then either measured directly or used comparatively
with another sample of glass.
5. Significance and Use
5.1.
|
Density, a fundamental property of glass, varies with changes
in composition and thermal history. Density determined either
by direct measurement or comparison is suggested if elemental
analysis is not available. It is impractical to measure density
on glass fragments smaller than two to three millimeters. If
two samples of glass can be differentiated by density, they
could not have originated from the same source. Further limited
discrimination may be possible by doing a density comparison
in conjunction with refractive index comparison. |
5.2.
|
Density may be used as a screening technique where large
numbers of larger fragments are encountered. This may be particularly
useful in identifying multiple sources present in the known
and/or questioned samples. |
6. Sample Handling
6.1.
|
Proper sample preparation is a prerequisite
for obtaining reliable results. Density measurements are not
appropriate for fragments of glass that contain inclusions or
cracks or when comparing a surface fragment to a bulk fragment.
|
6.2.
|
The variability of the known sample should be
determined when possible. An effort to sample the known glass
may include breaking a fragment of tempered glass and including
fragments from the bulk and surface to compare to the unknown. |
6.3.
|
Density determinations and comparisons between
known and questioned samples should be made using fragments
of approximately equal size. |
6.4.
|
The samples must be clean and dry
before proceeding. Cleaning may be accomplished in a number
of ways, such as rinsing with laboratory-grade detergent solution
followed by several rinses of deionized water in order to remove
dust and dirt. The samples must be dried following cleaning.
(See Section 6.5 in the Scientific Working Group for the
Materials Analysis Collection, Handling, and Identification
of Glass.) |
7. Analysis
7.1.
|
Density Gradient Column Method
(Kirk 1951) |
|
7.1.1. |
Scope |
|
|
|
The method involves placing, in a
vertical glass tube, a liquid containing a gradient of density.
The gradient is such that the density at any level is less than
that at any level lower in the tube and greater than that of
any level higher in the tube. When glass fragments are introduced
to the column, each will become suspended in the liquid at the
level that is the same density as that glass fragment. Fragments
of different density will settle to different levels in the
column. |
|
7.1.2.
|
Materials and setup |
|
|
Gradient tubes are usually 25cm to
45cm in length and 6mm to 18mm in diameter. A heavy liquid,
such as 1,4 dibromobenzene or bromoform, is mixed with a lighter
liquid, such as bromobenzene or ethanol, in varying proportions
to form a density gradient. For most purposes, about five layers
of liquids are used. The bottom layer of the density gradient
tube consists of heavy liquid only. The second layer consists
usually of three parts of the heavy liquid to one of the light.
The third consists of equal mixtures of heavy and light liquids.
The fourth layer is made of three parts light liquid and one
part heavy liquid. The top layer consists of light liquid only.
Each layer is added to the prior very slowly using a pipette
so as to not allow mixture at the interface. The bottom layer
is typically about a quarter of the total height of the column.
The second, third, and fourth layers should each be about half
the height of the first layer. The top layer should be the same
height as the bottom layer. The gradient tube should stand overnight
before being used so that the liquids will diffuse into each
other to form a gradient. |
|
7.1.3.
|
Use |
|
|
The fragments should be properly
documented prior to their addition into the density gradient
column so as to facilitate identification when they are recovered
from the gradient. The fragments to be compared are gently placed
in the density gradient and allowed to settle completely. The
position of the glass in the column may be better viewed using
back-illumination. Care should be taken to avoid changes in
the temperature of the column. |
|
|
|
|
|
7.2 |
Sink/Float Method |
|
7.2.1.
|
Comparative Method
|
|
|
7.2.1.1. Scope
|
|
|
The method involves suspending glass
fragments in a density solution within a constant temperature
bath. If two or more fragments are suspended, their densities
are the same. The numeric density value is not determined in
this method but can be determined using the absolute density
determination methods described in Section 7.2.2. of this guideline.
|
|
|
7.2.1.2. Materials and setup |
|
|
A temperature-controlled density column containing
a mixture of bromobenzene and bromoform is typically used.
|
|
|
7.2.1.3. Use |
|
|
Place the glass fragments to be compared
in the density column after temperature equilibrium has been
achieved. Add small amounts of the density liquids until at
least one fragment becomes suspended. |
|
7.2.2.
|
Absolute density determination
methods |
|
|
After glass fragments are suspended in a density solution
using the sink/float method, a numeric determination can be
made using the following methods.
7.2.2.1.
|
Analytical balance and plummet method
(Koons et al. 2002) |
|
7.2.2.1.1. Scope
The method involves measuring the suspending density
solution using an analytical balance and weighed plummet.
7.2.2.1.2. Materials and setup
An analytical balance capable of accurately determining
weights within 0.0001g with a tall weighing chamber
and a plummet are required. The plummet must sink in
a liquid whose density is above 2.6g/cm3 (Beverage and
Semen 1979). Analyze a reference glass sample of known
density prior to each use to ensure that the system
is operating within acceptable parameters.
7.2.2.1.3. Use
Determine the volume of the plummet by weighing it in
air and then in water at a known temperature. Place
the glass fragment in the density column and suspend
it using the density liquids. Weigh the plummet immersed
in the density column. Calculate the density of the
liquid and the glass (Koons et al. 2002).
|
|
|
7.2.2.2. |
Density meter method (Beveridge and Semen
1979) |
|
7.2.2.2.1. Scope
The method involves measuring the density of a suspending
liquid mixture using a density meter.
7.2.2.2.2. Materials and setup
A density meter with its sample chamber at the same constant
temperature as the density column is required (to +/-
0.1 degrees Celsius). Analyze a reference glass sample
of known density prior to each use to ensure that the
instrument is operating within acceptable parameters.
7.2.2.2.3. Use
A portion of the suspending density liquid mixture is
transferred into a calibrated density meter and the density
is recorded. |
|
8. Considerations
8.1.
|
Glass samples exhibit a range of density values.
Density is a function of the chemical composition of the glass
and its thermal history. The composition of a glass sample can
be measured by a variety of techniques. Thermal history is typically
assessed through a measurement of either refractive index or
density. |
8.2.
|
The variations in either refractive index or density at fixed
values of the other indicate that even with precise measurement
of one property, the other is capable of providing some degree
of additional discrimination. If density and refractive index
are used in conjunction when one parameter is measured, the
second gives an improvement in discrimination capability of
approximately twofold (Koons et al. 2002; Stoney and Thornton
1985). |
8.3.
|
Because there is a high correlation between density and refractive
index, a limited amount of additional significance can be placed
on an association based on the combination of these techniques. |
8.4.
|
Samples that are distinguished by density did not originate
from the same source.
|
9. References
Beveridge, A. D. and Semen, C. Glass density measurement using
a calculating digital density meter, Canadian
Society of Forensic Science Journal (1979) 12(3):113-116.
Kirk, P. L. Density and Refractive Index: Their Application
in Criminal Identification. American Lecture Series. Publication
112. American Lectures in Public Protection. Thomas, Springfield,
Illinois, 1951.
Koons, R. D., Buscaglia, J., Bottrell, M., and Miller, E. T. Forensic
glass comparisons. In: Forensic Science Handbook. 2nd ed.
R. Saferstein, ed. Prentice-Hall, Upper Saddle River, New Jersey,
2002, Volume 1, pp.161-213.
Stoney, D. A. and Thornton, J. I. The forensic significance of the correlation
of density and refractive index in glass evidence, Forensic
Science International (1985) 29:147-157.
|