Shattering the silence: How glass analysis speaks

• Jurisdiction: South Africa
• Published date: 16 August 2019
• Pages: 56-65
• Date: 16 August 2019
• Citation: (2005) 18 SACJ 56
• Author: Lirieka Meintjes-Van Der Walt

Shattering the silence:

How glass analysis speaks

LIRIEKA MEINTJES-VAN DER WALT

*

ABSTRACT

The article sets out to develop a ‘clear picture’ of how glass analysis may be

used for forensic purposes. It discusses the manufacture and properties of

different types of glass. The article examines glass breakage and the signif‌i cance

of fragments and fragment distribution in establishing the point, direction and

type of impact causing the breakage. Techniques of establishing the identity

of glass are analysed, eg the measurement of the refractive index of the glass

sample. Using the refractive index is still probably the most useful measurement

for the identif‌i cation of glass particles. The application of the Bayes Theorem to

glass analysis is also discussed. The evidentiary value of glass particles found at

a crime or accident scene is examined and possible errors and inadequacies are

pointed out. While modern analytical methods have extended the availability of

evidence, such testimony should be scrutinized carefully to ensure reliability

and cogency.

1 Introduction

Glass is a substance with innumerable uses as well as some qualities that

are unique. Apparently solid, it is in fact a liquid, cooled well below its

point of solidif‌i cation and held between two highly stretched sheets, a

fact that accounts for both its transparency and its tendency to shatter

into tiny fragments. Two thousand years ago the Roman historian Pliny

recorded his knowledge of the substance. According to Pliny, some sailors

were shipwrecked at the mouth of the river Syria, and there accidentally

discovered, when trying to light a f‌i re, crude glass that formed when the

heat of their f‌i re fused the sand of the shore with natron, a type of soda that

was in their pot.1 Little could they guess what the forensic possibilities of

their discovery would hold for future generations.

Glass can be found in many shapes, sizes, colours and types –– its uses

can vary from containers to optical devices. The varying composition

of glass allows it to be discriminated by physical, optical and elemental

characteristics.

56

* B Juris LLB (UPE) LLM (Rhodes) LLD (Leiden) Professor of Law and Research Director

of the South African Unit for Expert Evidence and Forensic Skills, Rhodes University,

Grahamstown.

1 JM Curran, TN Hicks and JS Buckleton Forensic Interpretation of Glass Evidence (2000)

1-2.

(2005) 18 SACJ 56

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