Okobu! Do Now: – Take out Notes and prepare for Quiz Agenda: – Review of Density techniques – Refractive Index HW: – Ch 4 #3,5,7-10,19-22,28-35 due 11/1

Sink/Float method

Egg Demo

Why Measure Density? Can be used as a screening technique with large numbers of fragments. Useful in identifying multiple sources present in the known and/or questioned samples. It is nondestructive and an intensive property (not dependant on sample mass). Need to measure very precisely in parts per hundred or thousand or better.

Glass Density Density can be measured by: directly determining mass and volume (usually by displacement) comparison by flotation comparison using a density gradient column Density gradient column method: Fragments of different densities settle at different levels in the column of liquid of varying density.

Flotation Method precise and rapid method for comparing glass densities. 1.a glass particle is immersed in a liquid. 1.The density of the liquid is carefully adjusted –by the addition of small amounts of an appropriate liquid –until the glass chip remains suspended in the liquid medium. 1.At this point, the glass will have the same density as the liquid medium –can be compared to other relevant pieces of glass which will remain suspended, sink, or float.

Density by the Flotation Method A glass particle is immersed in a liquid. The density of the liquid is adjusted by the addition of small amounts of another liquid until the glass chip remains suspended. At this point, the glass will have the same density as the liquid medium and can be compared to other relevant pieces of glass which will remain suspended, sink, or float.

Important Physical Properties Light waves travel in air at a constant velocity until they penetrate another medium, such as glass or water, at which point they are suddenly slowed, causing the rays to bend. The bending of light waves because of a change in velocity is called refraction. Refractive index is the ratio of the velocity of light in a vacuum to that in the medium under examination.

Important Physical Properties For example, at 25 o C the refractive index of water is This means that light travels times faster in a vacuum than it does in water. Like density, refractive index is an intensive property and will serve to characterize a substance. Refractive index of glass varies with small changes in composition or by how it is manufactured.

refractive index = velocity of light in vacuum velocity of light in medium

Snell’s Law N=1.52 The higher the n, the more the light bends N=1.33

Refractive Index By Immersion Immersing a glass particle in a liquid medium (silicone oil) whose refractive index can varied with temperature until it is equal to that of the glass particle. At this point, known as the match point, the Becke line disappears and minimum contrast between liquid and particle is observed: RI oil = RI glass. The Becke line is a bright halo near the boarder of a particle that is immersed in a liquid of a different refractive index.

Refractive Index By Immersion

When 2 substances have different RI, light passing through them produces a Becke Line A Becke line is a band or rim of light visible along a grain/crystal boundary in plane-polarized light.

Glass has higher refractive index-note white line inside Glass has lower refractive index-note white line outside Becke Lines

Manufacturing changed in late 1970’s making glass more uniform A rough statistical estimate of the likelihood of finding glass of that refractive before 1970 Problems with Refractive Index

The FBI has compiled density and refractive index data for glass from around the world. The FBI has identified a relationship between their refractive indices and densities for 1400 glass specimens that is better at classification. FBI Refractive Index vs Density Data

Do Now: Agenda: HW

Putting it Back Together Again? Examiners can fit together two or more pieces of glass that were broken from the same object. Because glass is amorphous, no two glass objects will break the same way.

Glass Transfer Evidence When glass objects are broken, glass flies backward from all parts of the object where cracks appear not just from point of impact. This creates a shower of minute glass particles and a transfer of evidence. Glass fragment comparison depends finding and measuring properties that will associate one glass fragment with another while eliminating other sources.

Collection of Glass Samples The glass fragments should be packaged in boxes to avoid further breakage. If evidence is to be examined for glass fragments, it should be taken whole and each item individually wrapped in paper and boxed. If even the remotest possibility exists that glass fragments may be pieced together, every effort must be made to collect all glass fragments. Submit glass evidence along with a representative sample of each type of glass from the crime scene.

Optical Properties of Glass Make side-by-side comparisons using similar- sized fragments. Place samples on a white surface using natural light. Use both fluorescent and incandescent light to determine the glass’s color. Visual color analysis is very subjective. Dyes and pigments can be almost impossible to extract.

Nonoptical Physical Properties of Glass Surface striations and markings –Rollers leave parallel ream marks on sheet glass –Markings may indicate the glass’s orientation when pieces are missing –Surface scratches, etchings, and other markings may also be used to individualize evidence Other Properties –Hardness=5-6 on Mohs scale; use a scratch test. –Determinations of curvature can distinguish flat glass from container, decorative, or ophthalmic.

Forensic Analysis of Glass

Quantitative Properties of Glass

Chemical Analysis of Glass Fluorescence –Under UV radiation, many glasses exhibit fluorescence (glow) – Caused by heavy metals (including tin) from “float” process or organic coatings Scanning Electron Microscopy Energy Dispersive X-ray Analysis –Can determine many elements simultaneously –Surfaces of samples (>50 mg) can be analyzed Atomic Absorption Spectroscopy –You must first know which elements are present –Can analyze ppm levels of elements present

Learning Check 1.Which unique chemical component would be found in each type of glass shown below? (beaker) (windshield) (crystal) 2. Use “Table 2.3” to determine if lead borosilicate glass can be distinguished from borosilicate glass by density, refractive index, or both.

Quantitative Properties of Glass

The Wave of the Future: Laser Ablation Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) –Laser burns off a microscopic sample –Elements are ionized by plasma –Detects 46 trace elements and their isotopes simultaneously in glass at < 1 ppb

LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) is a powerful analytical technology that enables highly sensitive elemental and isotopic analysis to be performed directly on solid samples. powerful analytical technology that enables highly sensitive elemental and isotopic analysis to be performed directly on solid samples.