OSAC 2022-S-0023, Standard Practice for the Forensic 

 Analysis of Explosives by Polarized Light Microscopy

OSAC 2022-S-0023 

Standard Guide for the 

Forensic Analysis of 

Explosives by Polarized Light 

Microscopy 

Ignitable Liquids, Explosives, and Gunshot Residue Subcommittee 

Chemistry: Trace Evidence Scientific Area Committee (SAC) 

Organization of Scientific Area Committees (OSAC) for Forensic Science 
OSAC 2022-S-0023, Standard Practice for the Forensic 

 Analysis of Explosives by Polarized Light Microscopy

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OSAC Proposed Standard 

OSAC 2022-S-0023 

Standard Guide for the Forensic 

Analysis of Explosives by Polarized 

Light Microscopy

Prepared by 

Ignitable Liquids, Explosives, and Gunshot Residue Subcommittee 

Version: 2.0 

July 2023 

Disclaimer: 

This OSAC Proposed Standard was written by the (subcommittee) of the Organization of Scientific Area 

Committees (OSAC) for Forensic Science following a process that includes an open comment period. This 

Proposed Standard will be submitted to a standards developing organization and is subject to change. 

There may be references in an OSAC Proposed Standard to other publications under development by 

OSAC. The information in the Proposed Standard, and underlying concepts and methodologies, may be 

used by the forensic-science community before the completion of such companion publications. 

Any identification of commercial equipment, instruments, or materials in the Proposed Standard is not a 

recommendation or endorsement by the U.S. Government and does not imply that the equipment, 

instruments, or materials are necessarily the best available for the purpose. 

To be placed on the OSAC Registry, certain types of standards first must be reviewed by a Scientific and 

Technical Review Panel (STRP). The STRP process is vital to OSAC’s mission of generating and 

recognizing scientifically sound standards for producing and interpreting forensic science results. The 

STRP shall provide critical and knowledgeable reviews of draft standards or of proposed revisions of 

standards previously published by standards developing organizations (SDOs) to ensure that the published 

methods that practitioners employ are scientifically valid, and the resulting claims are trustworthy. 

The STRP panel will consist of an independent and diverse panel, including subject matter experts, human 

factors scientists, quality assurance personnel, and legal experts, which will be tasked with evaluating the 

proposed standard based on a comprehensive list of science-based criteria. 
OSAC 2022-S-0023, Standard Practice for the Forensic 

 Analysis of Explosives by Polarized Light Microscopy

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For more information about this important process, please visit our website 

at: https://www.nist.gov/topics/organization-scientific-area-committees-forensic-science/scientifictechnical-review-panels. 
OSAC 2022-S-0023, Standard Practice for the Forensic 

 Analysis of Explosives by Polarized Light Microscopy

1. Scope 

1.1 This standard addresses the use of polarized light microscopy (PLM) to identify explosive-related 

compounds from intact explosives and post-blast residues containing unconsumed explosive compounds or 

their solid reaction products, and to isolate them for further analysis. 

1.2 This standard is intended for use by competent forensic science practitioners with the requisite formal 

education, discipline-specific training (see Practice E2917), and demonstrated proficiency to perform 

forensic casework (refer to the T/SWGFEX Suggested Guide for Explosives Analysis Training). 

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is 

the responsibility of the user of this standard to establish appropriate safety and health practices and 

determine the applicability of regulatory limitations prior to use. 

2. Referenced Documents 

2.1 ASTM Standards 

E620 Practice for Reporting Opinions of Scientific or Technical Experts 

E860 Practice for Examining and Preparing Items that are or may become Involved in Criminal or 

Civil Litigation 

E1732 Terminology Related to Forensic Science 

E2917 Practice for Forensic Science Practitioner Training, Continuing Education, and Professional 

Development Program 

E3196 Terminology Relating to the Examination of Explosives 

E3253 Practice for Establishing an Examination Scheme for Intact Explosives 

E3329 Practice for Establishing an Examination Scheme for Explosive Residues 

2.2 Other Resources 

Technical/Scientific Working Group for Fire and Explosion Analysis (T/SWGFEX) Suggested Guide for 

Explosive Analysis Training 

(https://www.nist.gov/system/files/documents/2018/09/21/twgfex_suggest_guide_for_explosive_analysis

_training.pdf ) 

3. Terminology 

3.1 Definitions - For definitions of terms used in this guide other than those listed in 3.2, see E1732 and 

E3196. 

3.2 Definitions of Terms Specific to This Standard:

3.2.1 alpha (α) - symbol representing the lowest of the three principal refractive indices of a biaxial crystal. 

3.2.2 anomalous interference colors, n – atypical polarization colors which occur due to variation with 

wavelength of the refractive index (very few substances display this characteristic). 

3.2.3 Becke line, n – a halo observed near the boundary of a transparent particle that is mounted in a medium 

that differs from the particle’s refractive index. 

3.2.4 Becke line method, n - method for determining the refractive index of a transparent particle relative 

to its mountant by noting the direction in which the Becke line moves when the focus is changed. 
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Discussion—The Becke line always moves toward the higher refractive index medium (particle or 

mountant) then the focus is raised, and towards the lower refractive index medium when the focus is 

lowered. At the point where the index of the particle matches the index of the mounting medium, the Becke 

line is no longer visible. The Becke line is generally viewed at a wavelength of 589 nm (the D line of 

sodium [nD]). 

3.2.5 beta (β) - symbol representing the intermediate principal refractive index of a biaxial crystal. 

3.2.6 biaxial, adj - an anisotropic crystal in the orthorhombic, monoclinic, or triclinic system with three 

principal refractive index directions (α, β, γ) and two optic axes that are isotropic. 

3.2.7 birefringence, n - the numerical difference between the maximum and minimum refractive indices of 

anisotropic substances. 

3.2.8 crystal system, n - crystals are classified according to their crystallographic axes length and the angles 

between them. There are six crystal systems: cubic, tetragonal, hexagonal, orthorhombic, monoclinic, and 

triclinic. All crystal systems are anisotropic except for the cubic system, which is isotropic. 

3.2.9 epsilon (ε) - any vibration direction in the plane of the c axis for uniaxial crystals. 

3.2.10 euhedral, adj - well-formed crystals bounded by plane faces. 

3.2.11 extinction, n - condition when an anisotropic substance appears dark when observed between crossed 

polarizers and occurs when the vibration directions of the specimen are parallel to the vibration directions 

of the polarizer and analyzer. 

3.2.12 extinction, oblique, n - a type of extinction in which the vibration directions are inclined at an angle 

relative to a crystal face. 

3.2.13 extinction, parallel, n - a type of extinction in which the vibration directions are parallel to the crystal 

faces. 

3.2.14 extinction, symmetrical, n - a type of extinction in which the vibration directions bisect a prominent 

interfacial angle of the crystal 

3.2.15 gamma (γ) - symbol representing the highest of the three principal refractive indices of a biaxial 

crystal. 

3.2.16 interference figure, n – pattern observed during conoscopic observation of an anisotropic material 

which consists of a combination of extinction positions and interference colors corresponding to the full 

cone of directions by which the sample is illuminated. 

Discussion—Conoscopic observations are typically made by inserting a Bertrand lens into the body tube or 

by removing an eyepiece and viewing down the body tube. 

3.2.17 fusion methods, n - A process in which compounds are heated on a microscope slide and observed 

via PLM during heating, melting, and, upon cooling, recrystallization of the melt. 

Discussion—Fusion methods frequently employ a temperature controlled hot stage capable of at least 300O

Celsius placed on the stage of a polarizing light microscope. 
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3.2.18 omega (ω) - any vibration direction in the plane of the a axis for uniaxial crystals. 

3.2.19 optic axial angle (2V), n - the acute angle between two optic axes of a biaxial crystal. 

3.2 20 optic axis, n - a direction of isotropic light propagation in an anisotropic crystal. Uniaxial crystals 

have one optic axis; biaxial crystals have two optic axes. 

3.2.21 optic sign, n - determined by the relationship of the refractive indices of a material. For uniaxial 

crystals, if ε > ω, the crystal is positive (+); if ω > ε, the crystal is negative (-). For biaxial crystals, if γ-β

> β-α, the crystal is positive (+); if γ-β < β-α, the crystal is negative (-). 

3.2 22 polymorphism, n - crystallization into two or more chemically identical but crystallographically 

distinct forms. 

3.2.23 relief, n - contrast between a particle or crystal and its media due to the difference between their 

refractive indices. The greater the numerical difference in refractive indices, the greater the relief. 

3.2.24 subhedral, adj - crystals with imperfectly developed faces. 

3.2.25 uniaxial, adj - an anisotropic crystal in the tetragonal or hexagonal system having one optic axis 

(isotropic direction) and either two (tetragonal) or three (hexagonal) directions which are alike and 

perpendicular to the direction of the optic axis. 

4. Summary of Practice 

4.1 Solid chemical components in samples are analyzed using a polarized light microscope (PLM). The 

optical crystallographic properties such as refractive index, birefringence, and morphology are determined. 

Determination of these properties, which are different for various explosive components and other chemical 

compounds, can aid in the identification of explosives and explosive residues submitted to the forensic 

laboratory. 

4.2 Optical crystallographic properties of an unknown component can be determined by either mounting 

the component in a refractive index liquid, recrystallizing the component from a small drop of solvent, or 

recrystallizing the component from its melt, i.e., recrystallization occurring when a melted compound is 

cooled. Recrystallized components can be isolated for further analysis using other analytical techniques 

such as Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, or scanning electron 

microscopy-energy dispersive x-ray spectroscopy (SEM-EDS). 

4.3 Water-soluble components, such as inorganic salts used as oxidizers, are dissolved in a drop of water 

and tested using reagents to form characteristic microcrystals. Microcrystal tests can aid in the 

determination of anions and cations present in oxidizers and other components of explosives. 

4.4 The optical crystallographic properties of an inorganic compound and microcrystal testing for its ions 

are independent techniques that provide complementary information. A combination of tests that includes 

the optical crystallographic characterization of a compound and microchemical tests to confirm the anion 

and cation are used to identify oxidizers and other salts present in explosives. 

4.5 A combination of tests that include optical crystallographic characterization or microchemical tests in 

conjunction with other independent techniques are used to identify chemical components present in 

explosives. Refer to ASTM E3253 and E3329 for the requirements for the identification of commonly 

encountered explosives and chemical components present in explosive materials. 
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5. Significance and Use 

5.1 This standard is designed to assist the analyst using polarized light microscopy for the examination of 

test samples for the presence of explosives. 

5.2 Intact explosives and post-blast residues that contain unconsumed explosive components or their solid 

reaction products are suitable for this standard (E3253, E3329). Particles only a fraction of a millimeter in 

size can be examined using this standard. 

5.3 This standard is not suitable for the examination of smokeless powder or liquid explosives. 

 

5.4 Some of the microscopical techniques described in this standard allow for the recovery of the specimen 

for subsequent analysis, but others, such as microcrystal tests, are destructive. 

5.5 Identifications of explosive-related components based on these properties shall be part of an analytical 

scheme as described in ASTM E3253 and E3229. 

6. Apparatus 

6.1 Polarized light microscope - A compound microscope with a rotating stage, polarizing filters, substage 

condenser, and compensator slot. 

Note 1—Most commercially available polarized light microscopes should be adequate for the forensic analysis of 

explosives and appropriate for use following this standard. 

6.2 Objectives - Strain-free typically 10X, 20X, and 40X. 

6.2.1 A 40X objective with a numerical aperture of 0.65 or higher is recommended to provide the widest 

view of the back focal plane for conoscopic observations. 

6.2.2 A 10X dispersion staining objective is required if performing dispersion staining. 

6.3 Compensator - A full wave compensator for use with the microscope. Optional compensators include 

quarter-wave plate and quartz wedge. 

6.4 Hot stage - A temperature controlled hot stage for use with the microscope with a range of 

approximately 30o

C to 300oC. 

6.5 Alcohol lamp 

7. Reagents and Materials 

7.1 Purity of Reagents—Reagent grade chemicals shall be used in all tests. It is intended that all reagents 

conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society. 

Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to 

permit its use without lessening the accuracy of the determination. 

7.2 10% platinum chloride (H2PtCl6) in water 

7.3 zinc uranyl acetate 

7.4 ammonium molybdate ((NH4)6Mo7O24.4H2O) 
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7.5 squaric acid

7.6 potassium iodide (KI) 

7.7 nitron sulfate 

7.8 strychnine sulfate

7.9 0.3% methylene blue in deionized water (Methylene Blue Reagent) 

7.10 20% potassium nitrate (KNO3) in deionized water 

7.11 saturated zinc sulfate (ZnSO4) in deionized water 

7.12 Toluene or methylene chloride 

7.13 Sodium hydroxide 

7.14 Deionized water 

7.15 Reference materials, including reagent grade chemicals and explosive materials, can be obtained from 

commercial and retail sources or directly from the distributor or manufacturer. 

7.16 Refractive index liquids with known temperature coefficients (dn/dt) and tolerance at nD

7.17 Microscope slides and coverslips 

7.18 Glass rod with a tapering tip of approximately 1 mm or less in diameter 

7.19 Wood toothpicks 

7.20 Glass ring with an approximate diameter of 10-12 mm and 5-10 mm in height 

7.219 Tungsten needles 

7.22 589 nm light filter 

8. Initial Microscopical Examination 

8.1. Preliminary assessment of the samples using visual and stereo microscopical examinations are 

conducted following E3253-21 and E3329-21. If preliminary examinations indicate that the sample is an 

explosive mixture or a component thereof, a portion of the sample is examined by PLM in order to obtain 

further information regarding the composition of the sample. 

8.2 One to three particles or grains (approximately 1 mg or less) are mounted on a glass microscope slide 

with a cover slip and dispersed in a refractive index oil. For post-blast evidence a similar quantity of residue 

material is mounted in the same manner. Any refractive index oil in the 1.400 to 1.800 range should be 

used, however, if preliminary examinations indicate a particular explosive material, e.g., black powder, 

ammonium nitrate-fuel oil (ANFO) or Composition C-4, the examiner should select a refractive index oil 

that will enable confirmation of characteristic optical crystallographic data based upon Tables 1 and 2. 
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8.3 The mounted sample is examined with a polarized light microscope at low magnification (e.g. 10X). 

The magnification is increased as needed to better view particles for examination. 

8.4 Charcoal, if present, will appear as irregularly shaped, black and opaque particles, and as brown flakes 

often with cellular features such as pits or cell walls (12). 

8.5 Most common explosives and oxidizers are colorless when viewed microscopically with transmitted 

plane polarized light. Crossed polarizers are used to determine if the crystals are isotropic or anisotropic. 

Interference colors are assessed to determine if the birefringence is low (less than ~0.01), moderate (~0.01-

0.10) or high (above ~0.10). 

8.6 The relative refractive indices of the crystals present in the specimen are determined using the Becke 

line method or dispersion staining. 

8.7 The morphology, relative refractive indices, and interference colors can indicate the presence of an 

explosive ingredient or chemical component. See Tables 1 and 2. 

8.7.1 If explosive related components are indicated, additional microscopical analysis is performed to 

further characterize the components via optical crystallography (Sections 9 & 10). 

8.7.2 If inorganic salts are indicated, additional microscopical analysis may be performed by microcrystal 

tests (Section 11). 

8.7.3 If no explosive related components are indicated, further analysis using microscopy or other 

instrumental analyses could be performed to characterize or identify the substance in the case sample. Such 

examinations are beyond the scope of this document. 

9. Optical Crystallography by Solvent Recrystallization 

9.1 Solvent recrystallization allows for a more detailed optical crystallographic characterization of a 

compound as it recrystallizes from a small drop of suitable solvent. 

9.2 Water recrystallization for inorganic oxidizers and salts 

9.2.1 A small drop (5-7 mm in diameter) of distilled or deionized water is placed on a microscope slide. 

1-3 particles or grains (approximately 1 mg or less) from the sample with similar appearance are added to 

the drop. If the specimen does not readily dissolve, it should be crushed and dispersed with the tip of a 

glass rod to aid dissolution. 

9.2.1.1 Alternatively, if a large amount of insoluble components are present, a portion of the specimen can 

be mixed with water and filtered to remove water insoluble components such as carbon, aluminum, and 

sulfur. A small drop of the filtered water extract is then placed on a microscope slide. 

9.2.2 The test drop is examined using the PLM and a low magnification objective, e.g., 10X. Crystals 

typically form at the edge of the drop first. Distorted or subhedral crystals, if formed, are pushed back into 

the center of the drop using a device such as a glass rod, toothpick or metal probe to produce characteristic 

euhedral crystals. Multiple water-soluble components can be present in the sample and, depending on their 

concentration and solubility, could recrystallize at different times as the drop evaporates. 

9.2.3 Crystals formed as the drop evaporates are examined using plane-polarized and crossed-polarized 

light. Observe their crystal habit, relief, and whether they are isotropic or anisotropic. 

9.2.4 For anisotropic euhedral crystals, extinction is determined to be parallel, symmetrical, or oblique. 
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9.2.5 Interference colors are assessed to determine if the birefringence is low (less than ~0.01), moderate 

(~0.01-0.10), or high (above ~0.10). 

9.2.6 Observable interference figures are used to determine if the crystal is uniaxial or biaxial and the optic 

sign. Interference figures can also be used to relate morphology of the crystal to its optical orientation, and, 

if the crystal is biaxial, estimate the optic axial angle (2V). 

9.2.7 The optical crystallographic properties of the crystals are documented by description, sketch, or 

photomicrographs. 

9.2.8 The optical crystallographic properties of the crystals formed are compared to recrystallized known 

reference materials or published references (6, 8). A tentative identification is made if the optical 

crystallographic properties of the specimen are the same as a given compound. Table 1 lists optical 

crystallographic properties for twelve common oxidizers. 

9.2.9 If two or more oxidizers recrystallize from the drop, their tentative identification is compared to the 

optical properties determined from examinations of the intact sample (Section 8 and 10) to assess if a 

metathetical reaction has occurred. If a metathetical reaction occurred, the optical properties of the intact 

crystals and those obtained from recrystallization will be different. 

9.2.10 Depending on the analytical scheme chosen, dried crystals can be 1) mounted in refractive index oils 

for refractive index determination (Section 10), 2) analyzed via microcrystal tests (Section 11), or 3) 

analyzed by other instrumental techniques (E3253, E3329) If the crystals cannot be further analyzed on 

the glass slide, a tungsten needle is used to transfer the crystals from the glass slide to another substrate 

such as a potassium bromide pellet for additional analysis. 

9.3 Organic solvent recrystallization for sulfur 

9.3.1 A small drop (5-7 mm in diameter) of a suitable solvent, such as toluene or methylene chloride, is 

placed on a microscope slide. One to three particles or grains (approximately 1 mg or less) of sample are 

added to the solvent drop. Sulfur, if present, will recrystallize as highly refractive, birefringent dipyramids 

(diamond-shaped) or form liquid drops that crystallize when touched with a tungsten needle. 

9.3.2 The optical crystallographic properties of the crystals are documented by description, sketch, or 

photomicrograph. 

9.3.3 The optical crystallographic properties of the crystals formed are compared to recrystallized known 

reference materials or published references (5, 6). A tentative identification is made if the optical 

crystallographic properties of the specimen are the same as a given compound. Table 1 lists optical 

crystallographic properties for sulfur. 

9.3.4 Depending on the analytical scheme chosen, dried crystals can be further analyzed by other analytical 

techniques (E3253, E3329). If the crystals cannot be further analyzed while on the glass slide, a tungsten 

needle is used to transfer crystals from the glass slide to another substrate such as carbon tape for additional 

analysis. 

10. Optical Crystallography Using Refractive Index Liquids 

10.1 Organic and inorganic compounds present in explosives are characterized by their optical 

crystallographic properties by mounting intact material in a liquid of known refractive index. 
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10.2 One to three particles or grains (approximately 1 mg or less) of the sample, or dried crystals obtained 

during recrystallization, as described in Section 9, are mounted in a refractive index oil in the 1.400 to 1.800 

range. If previous examinations indicate a particular compound, e.g., potassium nitrate or RDX, the 

examiner should select a refractive index oil that will provide characteristic optical crystallographic data 

based on Tables 1 and 2. 

10.3 The crystals are examined using PLM to determine the optical crystallographic properties such as 

refractive indices, birefringence, extinction type (parallel, symmetrical, or oblique), optic sign, and optic 

axial angle (2V). 

10.4 Refractive index can be determined using the Becke line method or dispersion staining. The specimen 

can be mounted in additional refractive index liquids to confirm one or more refractive indices of the 

indicated compound. 

10.5 The optical crystallographic properties of the specimen are compared to known reference materials or 

published references (e.g., 5, 6, 8, 12-16, 22). A tentative identification is made if the optical 

crystallographic properties of the specimen are the same as a given compound. Table 1 lists optical 

crystallographic properties for three fuels (sulfur, ascorbic acid and sucrose) and twelve common oxidizers. 

Table 2 lists optical crystallographic properties for four organic high explosives. 

Table 1. Optical Crystallographic Properties of Common Fuels & Oxidizers

Compound Crystal 

System 

Refractive 

Indices/ 

Optic Angle 

Habit Comments 

Sulfur (S) (12) orthorhombic 1.958 

2.038 

2.245 

2V = 68 O(+) 

dipyramids High relief in solvent. Larger crystals 

appear yellowish in color. 

Ascorbic Acid 

(22) 

monoclinic 1.474 

1.595 

1.746 

2V = 88O(+) 

 High order interference colors 

Sucrose (12, 

22) 

monoclinic 1.540 

1.567 

1.572 

2V = 48O(-) 

Sodium 

chlorate 

(NaClO3) (8) 

cubic 1.518 squares, 

rectangles 

Barium nitrate 

(Ba(NO3)2) (8) 

cubic 1.571 cubes 

octahedra 

Strontium 

nitrate 

(Sr(NO3)2) (6) 

cubic 1.586 octahedra 

Lead nitrate 

(Pb(NO3)2) (8) 

cubic 1.781 octahedra 

cubes 

High relief in water 
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Potassium 

perchlorate 

(KClO4) (8) 

orthorhombic 1.4731 

1.4737 

1.4769 

2V = 50O(+) 

rectangular 

prisms, rhombs 

Low order interference colors 

Ammonium 

perchlorate 

(NH4ClO4)(8) 

orthorhombic 1.4818 

1.4833 

1.4881 

2V = 70O(+) 

six-sided 

prisms 

Low order interference colors. Crystals 

lying on a {110} face will exhibit an 

off-centered biaxial interference figure. 

Sodium 

perchlorate 

(NaClO4) (8) 

orthorhombic 1.4606 

1.4617 

1.4730 

2V = 69O(+) 

See 

NaClO4.2H20 

Sodium chlorate recrystallizes as the 

hydrate. Optical data is for the 

anhydrate. 

Sodium 

perchlorate 

dihydrate 

(NaClO4.2H2O) 

(8) 

monoclinic diamonds, 

rhombs 

Low relief in water. Some orientations 

exhibit oblique extinction. Refractive 

indices not known. 

Potassium 

chlorate 

(KClO3) (8) 

monoclinic 1.415 

1.517 

1.523, 

2V = 28O(-) 

diamondshaped tablets, 

often truncated 

Moderate order interference colors, 

symmetrical extinction 

Potassium 

nitrate 

(KNO3)(8) 

hexagonal & 

orthorhombic 

1.3346 

1.5056 

1.5064 

2V = 7O (-) 

rhombohedra & 

chevrons early; 

prisms late 

The hexagonal forms with high order 

interference colors appear first at the 

edge of the drop. Moving them to the 

middle of the drop with a glass rod or 

toothpick causes the stable 

orthorhombic forms to appear. 

Ammonium 

nitrate 

(NH4NO3) (8, 

9) 

orthorhombic 1.413 

1.611 

1.637 

2V = 35O (-) 

prisms, as 

blades & rods 

Crystal formation is slow due to the 

high solubility of ammonium nitrate. 

The resulting crystals exhibit high 

order interference colors. As a fusion 

preparation cools, three to four 

polymorphs are observed. MP = 169C 

Sodium nitrate 

(NaNO3) (8) 

hexagonal 1.5874 (ω) 

1.3361 (ε) 

rhombohedra High order interference colors and 

symmetrical extinction. 

Table 2. Crystallographic and Optical Properties of High Explosives 

Compound Crystal System Optical Properties Habit Comments 

RDX (13) orthorhombic 1.578 

1.597 

1.602 

2V = 53O(-) 

 Exhibits anomalous interference 

colors and dispersed extinction. 

MP = 204-206C 

HMX (13) monoclinic 1.589 

1.594 

1.73 

2V = 20O (+) 

dipyramids & 

pinacoids, rare 

acicular 

Typically exhibits high order 

interference colors. 

Oblique extinction 

TNT (13) orthorhombic 1.543 

1.674 

1.717 

 MP = 81C 
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2V = 60 (-) 

PETN (13) tetragonal ω = 1.556 

ε = 1.551 

 MP = 141.5-142C 

11. Microcrystal tests 

11.1 Microcrystal tests are chemical precipitation reactions in which the target ion forms an insoluble 

reaction product with the reagent. The crystals formed are examined using a polarized light microscope. 

Microcrystal tests are used in conjunction with other independent techniques to identify explosive related 

compounds. 

11.2 Microcrystal tests are used to test for anions and cations of oxidizers and other substances such as 

aluminum that are present in explosives. Many microcrystal tests are available, those commonly used in 

explosives analysis are listed in Table 3. 

11.3 The presence of additional water-soluble compounds can interfere with, or affect the formation of, 

microcrystals of the target anion or cation. If this occurs, further isolate the target compound and repeat 

the microcrystal test or use a different analytical technique. 

11.4 Prior to casework, the reagents used for the microcrystal tests are tested for reliability using the 

appropriate positive and negative controls. The crystals obtained with the positive control are compared 

to the published references (1, 4, 7, 17, 19, 20, 21). 

11.5 There are four routine methods for conducting microcrystal tests (4). Test drops and reagent drops 

used for microcrystal tests should be approximately 5-7 mm in diameter or smaller. Most reactions take 

place within one minute. Test drops with lower concentrations may take longer to occur. 

Note 2—The methods described below could have different number designations as found in reference 4. 

11.5.1 Method 1

11.5.1.1 A drop of deionized water is placed on a clean microscope slide and one or more particles of the 

sample is added with stirring, crushing and dispersing as needed to dissolve the water-soluble components. 

The test drop should be kept small. Alternatively, 1-5 grains or particles of the sample are placed in a spot 

well plate or test tube and 3-4 drops of deionized water are added. The specimen is stirred with a glass rod 

to disperse and dissolve any water-soluble components. A test drop of the aqueous solution is placed on a 

clean microscope slide. 

11.5.1.2 Place a drop of reagent near, but not touching, the test drop. Depending on the microcrystal test 

being conducted some reagent solutions are pre-made and a drop added to the slide. Other reagent drops 

are created on the slide by placing a drop of water near the test drop and adding 1-5 particles of the reagent 

to the water droplet. 

11.5.1.3 The test drop and reagent drop are drawn together using a clear glass rod or toothpick. The 

preparation is immediately examined microscopically using a 10X objective. Higher magnification can be 

used but care must be taken to ensure the objective does not contact the test drop. 

11.5.1.4 The preparation is examined for the presence of characteristic microcrystals or the lack of a 

reaction. 

11.5.1.5 The formation of microcrystals corresponding to those obtained with the positive control indicates 

the presence of that anion or cation. 
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11.5.2 Method 2 

11.5.2.1 Follow 11.5.1 except the reagent drop is added to the middle of the test drop. 

11.5.2.2 The preparation is examined microscopically as described in Method 1 for the presence of 

characteristic microcrystals or the lack of a reaction. 

11.5.2.3 The formation of microcrystals corresponding to those obtained with the positive control indicates 

the presence of that anion or cation. 

11.5.3 Method 3 

11.5.3.1 Follow 11.5.1 except add reagent particles into the middle of the test drop. 

11.5.3.2 The preparation is examined microscopically as described in Method 1 for the presence of 

characteristic microcrystals or the lack of a reaction. 

11.5.3.3 The formation of microcrystals corresponding to those obtained with the positive control indicates 

the presence of that anion or cation. 

11.5.4 Method 4 

11.5.4.1. A small drop of deionized water is placed on a clean microscope slide. One or more particles of 

the sample is added to the drop with stirring, crushing, and dispersing as needed to dissolve the watersoluble components. The test drop should be kept small. 

11.5.4.2 A glass ring with an internal diameter of about 10-12 mm is placed around the test drop so the 

drop is in the center of the ring. 

11.5.4.3 A small drop of the prepared reagent is placed on a coverslip. 

11.5.4.4 A small drop of the volatilizing solution (e.g., 10% sodium hydroxide) is added to the test drop 

inside the center of the ring. 

11.5.4.5 The coverslip is immediately placed onto the glass ring such that the reagent drop is suspended in 

the center of the ring above the sample. 

11.5.4.6 The slide preparation is placed on the microscope stage ensuring sufficient working distance 

between the top of the glass ring and the objective. 

11.5.4.7 The reagent drop is examined for the presence of microcrystals or the lack of reaction. 

11.5.4.8 The formation of microcrystals corresponding to those obtained with the positive control indicates 

the presence of that anion or cation. 

11.6 Table 3 lists twelve tests for anions and cations common to low explosives along with the reagent, 

literature reference, and method as described in 11.5. Microcrystal tests for additional ions can be found in 

Chamot & Mason (4), Benedetti-Pichler (21), Wills (19), and Hollifield (20). 

Table 3. Microcrystal Tests for Ions 

Cation/anion tested Microcrystal test & reference Method 

Ammonium Chloroplatinic acid (4) 4 

Potassium Chloroplatinic acid (4) 1, if positive, repeat using Method 4. If 

potassium present and ammonium 

absent, Method 4 test will be negative 

Sodium zinc uranyl acetate (4) 1 

Aluminum ammonium molybdate (4) 2 
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Magnesium Squaric acid (19, 20) 1 

Barium Squaric acid (19, 20) 1 

Strontium Squaric acid (19, 20) 1 

Lead Potassium iodide (4) 1 

Nitrate Nitron sulfate (1, 4) 1 

Perchlorate Strychnine sulfate (4) 1, 3 

Perchlorate Methylene Blue (7) 1 

12. Fusion Methods 

12.1 Fusion methods (10) are techniques that allow for the characterization of crystals by determining the 

melting point (MP) and observing optical crystallographic properties as the sample recrystallizes from the 

melt. This method is most useful for organic high explosives (11, 14, 15, 16) but can also be applied to 

some inorganic explosives (9). Crystalline compounds examined using fusion methods must first be isolated 

from mixtures by physical or chemical separation. 

12.2 Examination Using Temperature Controlled Hot Stage 

12.2.1 The isolated crystalline substance is placed on a microscope slide with a cover slip, or on a cover 

slip (according to the hot stage manufacturer’s instructions). 

12.2.2 The preparation is placed in the hot stage at a temperature of approximately 50C. If a particular 

explosive compound has been indicated from prior analysis, a temperature approximately 10-20C lower 

than the expected melting point can be used. 

12.2.3 The specimen is brought into the field of view. The temperature of the hot stage is raised while the 

specimen is viewed using the polarized light microscope. The melting point is recorded. The temperature 

is slowly lowered while observing the recrystallization of the sample from the melt. 

12.2.4 The optical crystallographic properties such as morphology, crystal front shape, polymorphism, 

interference colors, anomalous interference colors, extinction characteristics, interference figure, and optic 

sign are observed for each solid state as appropriate. 

12.3 Alternatively, the sample is heated on a microscope slide using an alcohol lamp (9). This method is 

especially useful for substances with melting points above 300C, including most inorganic oxidizers. An 

accurate melting point is not determined, but the sample is observed through the microscope as the slide 

cools to obtain various optical crystallographic properties, as described in 12.2.4. 

12.4 Compare the optical crystallographic properties of the specimen to published reference descriptions 

(9-11, 14-16) or a known reference material. 

12.5 Mixed fusions 

12.5.1 If an explosive compound is indicated using optical crystallography, microcrystal tests, or other 

techniques, a mixed fusion using a known reference material of the suspected compound can confirm the 

identity of the unknown (9, 10). 

12.5.2 A small amount of isolated crystalline substance from the sample is placed on a slide, under a 

coverslip, but near the edge of the coverslip. The sample is melted over an alcohol lamp. The melted 

sample should fill approximately half of the area under the coverslip. 
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12.5.3 Once the unknown sample has cooled and recrystallized, the known reference material is placed near 

the opposite edge of the coverslip and melted over an alcohol lamp such that the melted standard flows 

under the coverslip and contacts the previously cooled unknown specimen. The portion of the unknown 

specimen that contacts the reference material will also melt during this process. 

12.5.4 Using the PLM, the zone of mixing is viewed as the preparation cools and recrystallizes. If the 

unknown compound is the same as the reference material, the crystals of the unknown will continue to grow 

through the zone of mixing with no change in crystal growth rate or in appearance. If the unknown 

compound and reference material are different, a change in the crystal growth rate occurs and the crystals 

in the zone of mixing usually appear different. 

12.5.5 Once cooled, the mixed fusion preparation can be inserted into the hot stage at a temperature 

approximately 10 degrees lower than the melting point of the reference material. While observing the zone 

of mixing, slowly raise the temperature (1-3 degrees/minute) of the hot stage to the melting point of the 

reference material If the two compounds are the same, the melting point will be the same across the zone 

of mixing. 

Note 3—This can only be performed if the sample melts below the upper temperature range of the hot stage. 

13. Documentation 

13.1 Retain all notes and supporting analytical data used for the identification or tentative identification of 

explosive related compounds in accordance with E620, E860 and E3255. Examples of such data include 

photomicrographs and detailed descriptions of microscopical observations. 

13.2 Record case notes in sufficient detail such that an independent analyst could understand and evaluate 

the work performed, independently interpret the data, and form an opinion. 

14. References 

1. Lawrence, G., and Fink, M., “The Evaluation of Nitron Sulfate as a Microchemical Test for Some 

Common Oxidizers,” The Microscope, Vol 55, No. 2, 2007, pp 55-58. 

2. Parker, R. G., Stephenson, M. O., McOwen, J. M., and Cherolis J. A., “Analysis of Explosives and 

Explosive Residues. Part 1: Chemical Tests, ” Journal of Forensic Science, Vol 20, No. 1, January 1975. 

3. Chamot E. M., and Mason C. W., Handbook of Chemical Microscopy, Volume 1 John Wiley & Sons, 

Inc., New York, NY, 1958, p. 51. 

4. Chamot, E., and Mason, C., Handbook of Chemical Microscopy, Volume II, John Wiley & Sons, Inc., 

New York, NY, 1940. 

5.Hopen, T. J., “Tricks of the Trade, Extraction and Identification of Sulfur,” The Microscope, Vol 48, No. 

1, 2000, p. 51. 

6. Winchell A. N., The Microscopic Characters of Artificial Inorganic Solid Substances or Artificial 

Minerals, John Wiley & Sons, Inc., New York, NY, 1931. 

7. Hopen T. J., and Crippin J. B., “Methylene Blue Microchemical for the Detection and Identification of 

Chlorates and Perchlorates,” The Microscope, Vol 49, 2001. 

8. Hopen T.J., and Kilbourn J. H., “Characterization and Identification of Water Soluble Explosives,” The 

Microscope, Vol 33, No. 1, 1985. 
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Analysis of Explosives by Polarized Light Microscopy

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9. Kilbourn J. H., and McCrone W. C., “Fusion Methods Identification of Inorganic Explosives,” The 

Microscope, Vol 33, No. 2, 1985. 

10. McCrone W. C., Fusion Methods in Chemical Microscopy: A Textbook and Laboratory Manual, 

Interscience Publishers, Inc., New York, NY, 1957. 

11. McCrone W. C., “The Identification of High Explosives by Microscopic Fusion Methods,” 

Microchemical Journal, Vol 3, 1959, pp 479-490. 

12. McCrone W. C., and Delly J. G., The Particle Atlas, Edition 2, Volumes I, II & IV, Ann Arbor Science 

Publishers, Inc., Ann Arbor, MI, 1973. 

13. McCrone W. C., Delly J. G., and Palenik S.J., The Particle Atlas, Edition 2, Volume V, Ann Arbor 

Science Publishers, Inc., Ann Arbor, MI, 1979. 

14. McCrone W. C., Andreen J. H., and Tsang S. M. “Identification of Organic High Explosives,” The 

Microscope, Vol 41, 1993, pp 161-182. 

15. McCrone W. C., Andreen J. H., and Tsang S. M. “Identification of Organic High Explosives, II,” The 

Microscope, Vol 42, 1994, pp 61-73. 

16. McCrone W. C., Andreen J. H., and Tsang S. M. “Identification of Organic High Explosives III,” The 

Microscope, Vol 42, 1999, pp 183-200. 

17. McCrone, W.C., “Solubility, Recrystallization and Microchemical Tests on Nanogram Single 

Particles,” The Microscope, V34 N2, 1986, pp 107-118. 

18. Delly J. G., Essentials of Polarized Light Microscopy and Ancillary Techniques, Hooke College of 

Applied Sciences, Westmont, IL, 2017. 

19. Wills WF, Jr., “Squaric Acid Revisited,” The Microscope, Vol 38, 1990. 

20. Hollifield, J.M., “Characterization of Squaric Acid Precipitates,” The Microscope, V51 N2, 2003, pp 

81-103. 

21. Benedetti-Pichler, A.A., “Identification of Materials via Physical Properties, Chemical Tests, and 

Microscopy”, Academic Press Inc., New York, NY, 1964. 

22. Winchell A. N., The Optical Properties of Organic Compounds, McCrone Research Institute, Chicago, 

IL, 1987 (reprint of 1942 edition, Academic Press, Inc) 

23. New York Microscopical Society, Glossary of Microscopical Terms and Definitions, Second Edition, 

McCrone Research Institute, Chicago, IL, 1989.