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

Analysis of Explosives by Polarized Light Microscopy

2

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: 1.0

February 2022

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 Guide for the Forensic

Analysis of Explosives by Polarized Light Microscopy

3

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 Guide for the Forensic

Analysis of Explosives by Polarized Light Microscopy

1 Standard Guide for the Forensic Analysis of Explosives By Polarized Light Microscopy

2 __________________________________________________________________________

3

4 1. Scope 

5

6 1.1 This standard addresses the use of polarized light microscopy (PLM) to identify explosive-related 

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

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

9 1.2 This standard is intended for use by competent forensic science practitioners with the requisite formal 

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

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

12 1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is 

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

14 determine the applicability of regulatory limitations prior to use.

15 2. Referenced Documents

16

17 2.1 ASTM Standards

18 E620 Practice for Reporting Opinions of Scientific or Technical Experts

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

20 Civil Litigation

21 E1732 Terminology Related to Forensic Science

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

23 Development Program

24 E3196 Terminology Relating to the Examination of Explosives

25 E3253 Practice for Establishing an Examination Scheme for Intact Explosives

26 E3329 Practice for Establishing an Examination Scheme for Explosive Residues

27

28 2.2 Other Resources

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

30 Explosive Analysis Training 

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

32 _training.pdf)

33

34 3. Terminology

35

36 3.1 Definitions - For definitions of terms used in this guide other than those listed in 3.2, see E1732 and 

37 E3196.

38

39 3.2 Definitions of Terms Specific to This Standard:

40

41 3.2.1 alpha (α) - symbol representing the lowest of the three principal refractive indices of a biaxial 

42 crystal.

43

44 3.2.2 anomalous interference colors, n – atypical polarization colors which occur due to variation with 

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

46

47 3.2.3 Becke line, n – a halo observed near the boundary of a transparent particle that is mounted in a 

48 medium that differs from the particle’s refractive index. 
2

49

50 3.2.4 Becke line method, n - method for determining the refractive index of a transparent particle relative 

51 to its mountant by noting the direction in which the Becke line moves when the focus is changed. 

52

53 Discussion—The Becke line always moves toward the higher refractive index medium (particle or 

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

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

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

57 of sodium [nD]). 

58

59 3.2.5 beta (β) - symbol representing the intermediate principal refractive index of a biaxial crystal.

60

61 3.2.6 biaxial, adj - an anisotropic crystal in the orthorhombic, monoclinic, or triclinic system with three 

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

63

64 3.2.7 birefringence, n - the numerical difference between the maximum and minimum refractive indices 

65 of anisotropic substances.

66

67 3.2.8 crystal system, n - crystals are classified according to their crystallographic axes length and the 

68 angles between them. There are six crystal systems: cubic, tetragonal, hexagonal, orthorhombic, 

69 monoclinic, and triclinic. All crystal systems are anisotropic except for the cubic system, which is 

70 isotropic.

71

72 3.2.9 epsilon (ε) - any vibration direction in the plane of the c axis for uniaxial crystals.

73

74 3.2.10 euhedral, adj - well-formed crystals bounded by plane faces.

75

76 3.2.11 extinction, n - condition when an anisotropic substance appears dark when observed between 

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

78 directions of the polarizer and analyzer.

79

80 3.2.12 extinction, oblique, n - a type of extinction in which the vibration directions are inclined at an 

81 angle relative to a crystal face.

82

83 3.2.13 extinction, parallel, n - a type of extinction in which the vibration directions are parallel to the 

84 crystal faces.

85

86 3.2.14 extinction, symmetrical, n - a type of extinction in which the vibration directions bisect a 

87 prominent interfacial angle of the crystal

88

89 3.2.15 gamma (γ) - symbol representing the highest of the three principal refractive indices of a biaxial 

90 crystal.

91

92 3.2.16 interference figure, n – pattern observed during conoscopic observation of an anisotropic material 

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

94 cone of directions by which the sample is illuminated.

95

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

97 or by removing an eyepiece and viewing down the body tube.

98
3

99 3.2.17 fusion methods, n - A process in which compounds are heated on a microscope slide and observed 

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

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

300O 102 Celsius placed on the stage of a polarizing light microscope.

103 3.2.18 omega (ω) - any vibration direction in the plane of the a axis for uniaxial crystals.

104 3.2.19 optic axial angle (2V), n - the acute angle between two optic axes of a biaxial crystal.

105

106 3.2 20 optic axis, n - a direction of isotropic light propagation in an anisotropic crystal. Uniaxial crystals 

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

108

109 3.2.21 optic sign, n - determined by the relationship of the refractive indices of a material. For uniaxial 

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

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

112

113 3.2 22 polymorphism, n - crystallization into two or more chemically identical but crystallographically 

114 distinct forms.

115

116 3.2.23 relief, n - contrast between a particle or crystal and its media due to the difference between their 

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

118

119 3.2.24 subhedral, adj - crystals with imperfectly developed faces.

120

121 3.2.25 uniaxial, adj - an anisotropic crystal in the tetragonal or hexagonal system having one optic axis 

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

123 perpendicular to the direction of the optic axis.

124

125 4. Summary of Practice

126

127 4.1 Solid chemical components in samples are analyzed using a polarized light microscope (PLM). The 

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

129 determined. Determination of these properties, which are different for various explosive components and 

130 other chemical compounds, can aid in the identification of explosives and explosive residues submitted to 

131 the forensic laboratory. 

132

133 4.2 Optical crystallographic properties of an unknown component can be determined by either mounting 

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

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

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

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

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

139

140 4.3 Water-soluble components, such as inorganic salts used as oxidizers, are dissolved in a drop of water 

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

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

143

144 4.4 The optical crystallographic properties of an inorganic compound and microcrystal testing for its ions 

145 are independent techniques that provide complementary information. A combination of tests that includes
4

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

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

148

149 4.5 A combination of tests that include optical crystallographic characterization or microchemical tests in 

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

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

152 encountered explosives and chemical components present in explosive materials.

153

154 5. Significance and Use

155

156 5.1 This standard is designed to assist the analyst using polarized light microscopy for the examination of 

157 test samples for the presence of explosives. 

158

159 5.2 Intact explosives and post-blast residues that contain unconsumed explosive components or their solid 

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

161 in size can be examined using this standard. 

162

163 5.3 This standard is not suitable for the examination of smokeless powder or liquid explosives. 

164 

165 5.4 Some of the microscopical techniques described in this standard allow for the recovery of the 

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

167

168 5.5 Identifications of explosive-related components based on these properties shall be part of an analytical 

169 scheme as described in ASTM E3253 and E3229.

170

171 6. Apparatus

172

173 6.1 Polarized light microscope - A compound microscope with a rotating stage, polarizing filters, 

174 substage condenser, and compensator slot. 

175

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

177 explosives and appropriate for use following this standard . 

178

179 6.2 Objectives - Strain-free typically 10X, 20X, and 40X. 

180 6.2.1 A 40X objective with a numerical aperture of 0.65 or higher is recommended to provide the widest 

181 view of the back focal plane for conoscopic observations. 

182 6.2.2 A 10X dispersion staining objective is required if performing dispersion staining.

183

184 6.3 Compensator - A full wave compensator for use with the microscope. Optional compensators include 

185 quarter-wave plate and quartz wedge.

186

187 6.4 Hot stage - A temperature controlled hot stage for use with the microscope with a range of 

approximately 30o

C to 300o 188 C.

189

190 6.5 Alcohol lamp

191

192 7. Reagents and Materials

193

194 7.1 Purity of Reagents—Reagent grade chemicals shall be used in all tests. It is intended that all reagents 

195 conform to the specifications of the Committee on Analytical Reagents of the American Chemical 
5

196 Society. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high 

197 purity to permit its use without lessening the accuracy of the determination.

198

199 7.2 10% platinum chloride (H2PtCl6) in water

200

201 7.3 zinc uranyl acetate

202

203 7.4 ammonium molybdate ((NH4)6Mo7O24.4H2O)

204

205 7.5 squaric acid

206

207 7.6 potassium iodide (KI)

208

209 7.7 nitron sulfate

210

211 7.8 strychnine sulfate

212

213 7.9 0.3% methylene blue in deionized water (Methylene Blue Reagent)

214

215 7.10 20% potassium nitrate (KNO3) in deionized water

216

217 7.11 saturated zinc sulfate (ZnSO4) in deionized water

218

219 7.12 Toluene or methylene chloride

220

221 7.13 Sodium hydroxide

222

223 7.14 Deionized water

224

225 7.15 Reference materials, including reagent grade chemicals and explosive materials, can be obtained 

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

227 7.16 Refractive index liquids with known temperature coefficients (dn/dt) and tolerance at nD

228

229 7.17 Microscope slides and coverslips

230

231 7.18 Glass rod with a tapering tip of approximately 1 mm or less in diameter

232

233 7.19 Wood toothpicks

234

235 7.20 Glass ring with an approximate diameter of 10-12 mm and 5-10 mm in height

236

237 7.219 Tungsten needles

238

239 7.22 589 nm light filter

240

241 8. Initial Microscopical Examination 

242 8.1. Preliminary assessment of the samples using visual and stereo microscopical examinations are 

243 conducted following E3253-21 and E3329-21. If preliminary examinations indicate that the sample is an 
6

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

245 further information regarding the composition of the sample.

246 8.2 One to three particles or grains (approximately 1 mg or less) are mounted on a glass microscope slide 

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

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

249 should be used, however, if preliminary examinations indicate a particular explosive material, e.g., black 

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

251 index oil that will enable confirmation of characteristic optical crystallographic data based upon Tables 1 

252 and 2. 

253 8.3 The mounted sample is examined with a polarized light microscope at low magnification (e.g. 10X). 

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

255 8.4 Charcoal, if present, will appear as irregularly shaped, black and opaque particles, and as brown flakes 

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

257 8.5 Most common explosives and oxidizers are colorless when viewed microscopically with transmitted 

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

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

260 (~0.01-0.10) or high (above ~0.10).

261 8.6 The relative refractive indices of the crystals present in the specimen are determined using the Becke 

262 line method or dispersion staining. 

263 8.7 The morphology, relative refractive indices, and interference colors can indicate the presence of an 

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

265 8.7.1 If explosive related components are indicated, additional microscopical analysis is performed to 

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

267 8.7.2 If inorganic salts are indicated, additional microscopical analysis may be performed by microcrystal 

268 tests (Section 11).

269 8.7.3 If no explosive related components are indicated, further analysis using microscopy or other 

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

271 Such examinations are beyond the scope of this document.

272 9. Optical Crystallography by Solvent Recrystallization

273 9.1 Solvent recrystallization allows for a more detailed optical crystallographic characterization of a 

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

275 9.2 Water recrystallization for inorganic oxidizers and salts

276 9.2.1 A small drop (5-7 mm in diameter) of distilled or deionized water is placed on a microscope slide.

277 1-3 particles or grains (approximately 1 mg or less) from the sample with similar appearance are added to 

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

279 glass rod to aid dissolution. 

280 9.2.1.1 Alternatively, if a large amount of insoluble components are present, a portion of the specimen 

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

282 and sulfur. A small drop of the filtered water extract is then placed on a microscope slide.
7

283 9.2.2 The test drop is examined using the PLM and a low magnification objective, e.g., 10X. Crystals 

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

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

286 characteristic euhedral crystals. Multiple water-soluble components can be present in the sample and, 

287 depending on their concentration and solubility, could recrystallize at different times as the drop 

288 evaporates.

289 9.2.3 Crystals formed as the drop evaporates are examined using plane-polarized and crossed-polarized 

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

291 9.2.4 For anisotropic euhedral crystals, extinction is determined to be parallel, symmetrical, or oblique. 

292 9.2.5 Interference colors are assessed to determine if the birefringence is low (less than ~0.01), moderate 

293 (~0.01-0.10), or high (above ~0.10). 

294 9.2.6 Observable interference figures are used to determine if the crystal is uniaxial or biaxial and the 

295 optic sign. Interference figures can also be used to relate morphology of the crystal to its optical 

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

297 9.2.7 The optical crystallographic properties of the crystals are documented by description, sketch, or 

298 photomicrographs. 

299 9.2.8 The optical crystallographic properties of the crystals formed are compared to recrystallized known 

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

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

302 crystallographic properties for twelve common oxidizers. 

303 9.2.9 If two or more oxidizers recrystallize from the drop, their tentative identification is compared to the 

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

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

306 crystals and those obtained from recrystallization will be different.

307 9.2.10 Depending on the analytical scheme chosen, dried crystals can be 1) mounted in refractive index 

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

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

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

311 such as a potassium bromide pellet for additional analysis. 

312 9.3 Organic solvent recrystallization for sulfur

313 9.3.1 A small drop (5-7 mm in diameter) of a suitable solvent, such as toluene or methylene chloride, is 

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

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

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

317 9.3.2 The optical crystallographic properties of the crystals are documented by description, sketch, or 

318 photomicrograph. 

319 9.3.3 The optical crystallographic properties of the crystals formed are compared to recrystallized known 

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

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

322 crystallographic properties for sulfur. 
8

323 9.3.4 Depending on the analytical scheme chosen, dried crystals can be further analyzed by other 

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

325 a tungsten needle is used to transfer crystals from the glass slide to another substrate such as carbon tape 

326 for additional analysis. 

327 10. Optical Crystallography Using Refractive Index Liquids

328 10.1 Organic and inorganic compounds present in explosives are characterized by their optical 

329 crystallographic properties by mounting intact material in a liquid of known refractive index. 

330 10.2 One to three particles or grains (approximately 1 mg or less) of the sample, or dried crystals obtained 

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

332 1.800 range. If previous examinations indicate a particular compound, e.g., potassium nitrate or RDX, the 

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

334 based on Tables 1 and 2. 

335 10.3 The crystals are examined using PLM to determine the optical crystallographic properties such as 

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

337 axial angle (2V). 

338 10.4 Refractive index can be determined using the Becke line method or dispersion staining. The 

339 specimen can be mounted in additional refractive index liquids to confirm one or more refractive indices 

340 of the indicated compound.

341 10.5 The optical crystallographic properties of the specimen are compared to known reference materials 

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

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

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

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

346

347 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
9

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

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.

348

349 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
10

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

2V = 60 (-)

MP = 81C

PETN (13) tetragonal ω = 1.556 

ε = 1.551

MP = 141.5-142C

350

351 11. Microcrystal tests

352 11.1 Microcrystal tests are chemical precipitation reactions in which the target ion forms an insoluble 

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

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

355 compounds. 

356 11.2 Microcrystal tests are used to test for anions and cations of oxidizers and other substances such as 

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

358 explosives analysis are listed in Table 3. 

359 11.3 The presence of additional water-soluble compounds can interfere with, or affect the formation of,

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

361 the microcrystal test or use a different analytical technique.

362 11.4 Prior to casework, the reagents used for the microcrystal tests are tested for reliability using the 

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

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

365 11.5 There are four routine methods for conducting microcrystal tests (4). Test drops and reagent drops 

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

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

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

369 11.5.1 Method 1

370 11.5.1.1 A drop of deionized water is placed on a clean microscope slide and one or more particles of the 

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

372 components. The test drop should be kept small. Alternatively, 1-5 grains or particles of the sample are 

373 placed in a spot well plate or test tube and 3-4 drops of deionized water are added. The specimen is 

374 stirred with a glass rod to disperse and dissolve any water-soluble components. A test drop of the 

375 aqueous solution is placed on a clean microscope slide.

376 11.5.1.2 Place a drop of reagent near, but not touching, the test drop. Depending on the microcrystal test 

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

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

379 reagent to the water droplet. 

380
11

381 11.5.1.3 The test drop and reagent drop are drawn together using a clear glass rod or toothpick. The 

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

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

384 11.5.1.4 The preparation is examined for the presence of characteristic microcrystals or the lack of a 

385 reaction. 

386 11.5.1.5 The formation of microcrystals corresponding to those obtained with the positive control

387 indicates the presence of that anion or cation. 

388

389 11.5.2 Method 2

390 11.5.2.1 Follow 11.5.1 except the reagent drop is added to the middle of the test drop.

391 11.5.2.2 The preparation is examined microscopically as described in Method 1 for the presence of 

392 characteristic microcrystals or the lack of a reaction.

393 11.5.2.3 The formation of microcrystals corresponding to those obtained with the positive control

394 indicates the presence of that anion or cation. 

395

396 11.5.3 Method 3

397 11.5.3.1 Follow 11.5.1 except add reagent particles into the middle of the test drop.

398 11.5.3.2 The preparation is examined microscopically as described in Method 1 for the presence of 

399 characteristic microcrystals or the lack of a reaction.

400 11.5.3.3 The formation of microcrystals corresponding to those obtained with the positive control

401 indicates the presence of that anion or cation. 

402

403 11.5.4 Method 4

404 11.5.4.1. A small drop of deionized water is placed on a clean microscope slide. One or more particles of 

405 the sample is added to the drop with stirring, crushing, and dispersing as needed to dissolve the water406 soluble components. The test drop should be kept small.

407 11.5.4.2 A glass ring with an internal diameter of about 10-12 mm is placed around the test drop so the 

408 drop is in the center of the ring.

409 11.5.4.3 A small drop of the prepared reagent is placed on a coverslip.

410 11.5.4.4 A small drop of the volatilizing solution (e.g., 10% sodium hydroxide) is added to the test drop 

411 inside the center of the ring.

412 11.5.4.5 The coverslip is immediately placed onto the glass ring such that the reagent drop is suspended 

413 in the center of the ring above the sample.

414 11.5.4.6 The slide preparation is placed on the microscope stage ensuring sufficient working distance 

415 between the top of the glass ring and the objective.

416 11.5.4.7 The reagent drop is examined for the presence of microcrystals or the lack of reaction.

417 11.5.4.8 The formation of microcrystals corresponding to those obtained with the positive control

418 indicates the presence of that anion or cation. 

419

420 11.6 Table 3 lists twelve tests for anions and cations common to low explosives along with the reagent, 

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

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

423

424 Table 3. Microcrystal Tests for Ions
12

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

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

425

426 12. Fusion Methods

427 12.1 Fusion methods (10) are techniques that allow for the characterization of crystals by determining the 

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

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

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

431 isolated from mixtures by physical or chemical separation.

432 12.2 Examination Using Temperature Controlled Hot Stage

433 12.2.1 The isolated crystalline substance is placed on a microscope slide with a cover slip, or on a cover 

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

435 12.2.2 The preparation is placed in the hot stage at a temperature of approximately 50C. If a particular 

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

437 than the expected melting point can be used.

438 12.2.3 The specimen is brought into the field of view. The temperature of the hot stage is raised while the 

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

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

441 12.2.4 The optical crystallographic properties such as morphology, crystal front shape, polymorphism, 

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

443 optic sign are observed for each solid state as appropriate. 

444 12.3 Alternatively, the sample is heated on a microscope slide using an alcohol lamp (9). This method is 

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

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

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

448 12.4 Compare the optical crystallographic properties of the specimen to published reference 

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

450 12.5 Mixed fusions 
13

451 12.5.1 If an explosive compound is indicated using optical crystallography, microcrystal tests, or other 

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

453 identity of the unknown (9, 10). 

454

455 12.5.2 A small amount of isolated crystalline substance from the sample is placed on a slide, under a 

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

457 sample should fill approximately half of the area under the coverslip.

458 12.5.3 Once the unknown sample has cooled and recrystallized, the known reference material is placed 

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

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

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

462 12.5.4 Using the PLM, the zone of mixing is viewed as the preparation cools and recrystallizes. If the 

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

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

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

466 in the zone of mixing usually appear different. 

467 12.5.5 Once cooled, the mixed fusion preparation can be inserted into the hot stage at a temperature 

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

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

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

471 zone of mixing. 

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

473 13. Documentation

474 13.1 Retain all notes and supporting analytical data used for the identification or tentative identification of 

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

476 photomicrographs and detailed descriptions of microscopical observations.

477 13.2 Record case notes in sufficient detail such that an independent analyst could understand and evaluate 

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

479 14. References

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

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

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

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

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

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

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

487 New York, NY, 1940.

488 5.Hopen, T. J., “Tricks of the Trade, Extraction and Identification of Sulfur,” The Microscope, Vol 48, 

489 No. 1, 2000, p. 51.
14

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

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

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

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

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

495 Microscope, Vol 33, No. 1, 1985.

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

497 Microscope, Vol 33, No. 2, 1985. 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

515 Applied Sciences, Westmont, IL, 2017.

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

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

518 81-103.

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

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

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

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

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

524 McCrone Research Institute, Chicago, IL, 1989.