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

Analysis of Explosives By Polarized Light Microscopy

OSAC 2022-S-0023

Standard Practice 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

2

Draft OSAC Proposed Standard

OSAC 2022-S-0023

Standard Practice for the Forensic 

Analysis of Explosives By Polarized 

Light Microscopy

Prepared by

Ignitable Liquids, Explosives, and Gunshot Residue Subcommittee

Version: 1.0

February 2021

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 Standard Practice for the Forensic Analysis of Explosives By Polarized Light Microscopy

2 __________________________________________________________________________

3

4 1. Scope 

5

6 1.1 This practice 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 and to isolate them for further analysis.

9 1.2 This practice 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 practice does not purport to address all of safety concerns, if any, associated with its use. It is the 

13 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 WK56998 Terminology Relating to the Examination of Explosives

25 WK67862 Practice for Establishing an Examination Scheme for Intact Explosives 

26

27 2.2 Other Resources

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

29 Explosive Analysis Training 

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

31 _training.pdf)

32

33 3. Terminology

34

35 3.1 Definitions - For definitions of terms used in this guide other than those listed in 3.2, see E1732 and 

36 WK56998.

37

38 3.2 Definitions of Terms Specific to This Standard:

39

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

41

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

43 wavelength of the refractive index; very few substances display this characteristic. 

44

45 Becke line, n – a halo observed near the boundary of a transparent particle when mounted in a medium 

46 that differs from its refractive index. 

47
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48 Becke line method, n - method for determining the refractive index of a transparent particle relative to its 

49 mountant by noting the direction in which the Becke line moves when the focus is changed. 

50

51 Discussion--The Becke line always moves toward the higher refractive index medium (particle or 

52 mountant) when the focus is raised (stage is lowered) ,and towards the lower refractive index medium 

53 when the focus is lowered (stage is raised). At the point where the index of the particle matches the 

54 index of the mounting medium, the Becke line is no longer visible. The Becke line is generally viewed at 

55 a wavelength of 589 nm (the D line of Sodium [nD]). 

56

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

58

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

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

61

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

63 anisotropic substances: ε − ω for uniaxial crystals and γ − α for biaxial crystals

64

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

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

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

68

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

70

71 euhedral, adj - well-formed crystals bounded by plane faces

72

73 eutectic, n - the lowest melting mixture of two or more substances miscible in the melt state.

74

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

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

77 of the polarizer and analyzer.

78

79 extinction, oblique, n - a type of extinction in which the vibration directions are oblique to the long 

80 direction of the crystal

81

82 extinction, parallel, n - a type of extinction in which the vibration directions are parallel and perpendicular 

83 to the long direction of the crystal

84

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

86 interfacial angle of the crystal

87

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

89

90 interference figure, n – pattern observed during conoscopic examination of an anisotropic material, which 

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

92 directions by which the sample is illuminated.

93

94 fusion methods, n— A process in which compounds are heated on a microscope slide and observed via 

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

96 Discussion—The procedure frequently employs a temperature controlled hot stage capable of at least 

300O 97 Celsius placed on the stage of a polarizing light microscope.
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98 omega (ω) - Any vibration direction in the plane of the a axes for uniaxial crystals

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

100

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

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

103

104 optic sign, n - determined by the relationship of the refractive indices of a material; for uniaxial crystals: 

105 if ε > ω, the crystal is positive (+); if ω > ε, the crystal is negative (-); for biaxial crystals: if γ-β > β-α, the 

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

107

108 polymorphism, n - crystallization into two or more chemically identical but crystallographically distinct 

109 forms.

110

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

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

113

114 subhedral, adj - crystals with imperfectly developed faces

115

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

117 direction) and either two (tetragonal) or three (hexagonal) directions which are alike and perpendicular to 

118 the direction of the optic axis.

119

120 4. Summary of Practice

121

122 4.1 Solid chemical components present in explosives are analyzed using a polarized light microscope 

123 (PLM). The optical crystallographic properties such as refractive index, birefringence, and morphology 

124 are determined. Determination of these properties, which are different for the various explosive 

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

126 laboratory. 

127

128 4.2 Optical crystallographic properties of an unknown component can be determined by mounting in a 

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

130 from its melt, i.e., recrystallization occurring when a melted compound is cooled. Recrystallized 

131 components can be isolated for further analysis using other analytical techniques such as Fourier 

132 transform infrared spectroscopy (FTIR), Raman spectroscopy, or scanning electron microscopy-energy 

133 dispersive electron spectroscopy (SEM-EDS).

134

135 4.3 Water-soluble components present in explosives, such as inorganic salts used as oxidizers, are 

136 dissolved in water and tested using reagents to form characteristic microcrystals. Microcrystal tests can 

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

138

139 4.4 The optical crystallographic properties of a compound and microchemical testing for its ions are 

140 independent techniques that provide complementary information. A combination of tests that include the 

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

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

143

144 4.5 A combination of tests that include optical crystallographic characterization or microchemical tests 

145 are used in conjunction with other independent techniques such as FTIR or SEM-EDS to identify 

146 chemical components present in explosives. Refer to ASTM WK67892 for the requirements for the 
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147 identification of commonly encountered explosives and chemical components present in explosive 

148 materials.

149

150 5. Significance and Use

151

152 5.1 This practice is designed to assist the analyst using polarized light microscopy for the identification of 

153 explosives. 

154

155 5.2 Intact explosives and post-blast residues that contain unconsumed explosive components are suitable 

156 for this practice. 

157

158 5.3 Polarized light microscopy is not appropriate for the examination of smokeless powder. 

159 

160 5.4 Some of the techniques described in this practice allow for the recovery of the specimen, but others 

161 such as microcrystal tests are destructive.

162

163 6. Apparatus

164

165 6.1 Polarized light microscope - A compound microscope with a rotating stage, polarizing filters, 

166 condenser and compensator slot. Most commercially available polarized light microscopes should be 

167 adequate for the forensic analysis of explosives. 

168 6.2 Objectives - Strain-free 10X, 20X and 40X. A 40X objective with a numerical aperture of 0.65 or 

169 higher is recommended to provide the widest view of the back focal plane for conoscopic observations.

170 6.3 Compensator - A full wave compensator for use with the microscope.

171 6.2 Hot stage - a temperature controlled hot stage for use with a microscope with a range of 

approximately 30oC to 300o 172 C.

173 6.5 The best practice for documenting some microscopical observations, such as crystal formations, is to 

174 take a digital photograph. It is advised that the minimum equipment required also has the capability of 

175 digital photography.

176 6.6 Alcohol lamp 

177 6.7 Glass ring with a diameter of approximately 10-12 mm and 5-10 mm in height

178

179 7. Reagents and Materials

180

181 7.1 Microscope slides and cover slips

182 7.2 Refractive index oils

183 7.3 Reagents for microcrystal tests: chloroplatinic acid, zinc/uranyl acetate, ammonium molybdate, 

184 squaric acid, potassium iodide, nitron sulfate, strychnine sulfate, methylene blue

185 7.4 Reference materials, including reagent grade chemicals and explosive materials, can be obtained from 

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

187 7.5 Distilled or deionized water

188 7.6 Toluene or methylene chloride, reagent grade

189 7.7 Sodium hydroxide, reagent grade

190 7.8 Glass rod with a tapering tip of approximately 1 mm or less in diameter

191 7.9 Glass ring with an approximate diameter of 10-12 mm

192 7.10 Tungsten needles

193 7.11 589 nm light filter

194

195 8. Initial Microscopical Examination 
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196 8.1. If preliminary examinations, such as stereomicroscopical observations, ignition susceptibility test 

197 (IST), and chemical spot tests indicate that the specimen is an explosive mixture or a component thereof, 

198 1-3 particles or grains (approximately 1 mg or less) are mounted on a glass microscope slide with a cover 

199 slip and dispersed in a refractive index oil. Any refractive index oil in the 1.400 to 1.660 range can be 

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

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

202 that will provide characteristic optical crystallographic data based upon Tables 1 and 2. 

203 8.2 Examine the mounted sample using a polarized light microscope using a 10X objective. Increase 

204 magnification as needed. 

205 8.3 Charcoal, if present, will appear as irregularly shaped, black and opaque particles, and as brown flakes 

206 often with cellular features such as pits or cell walls.

207 8.4 Most common explosives and oxidizers are colorless when viewed with a transmitted light 

208 microscope. Using crossed polarizers, determine if the crystals are isotropic or anisotropic. Assess the 

209 interference colors to determine if the birefringence is low (less than 0.01), moderate (0.01-0.10) or high 

210 (0.10 and above).

211 8.5 Determine relative refractive indices of crystals present in the sample using the Becke line method. 

212 8.6 The morphology, relative refractive indices and interference colors can indicate the presence of an 

213 explosive ingredient or chemical component. If explosive related components are indicated, additional 

214 microscopical analysis is performed to further characterize the components via optical crystallography. If 

215 inorganic salts are indicated, additional microscopical analysis can also be performed by microcrystal 

216 tests.

217 9. Optical Crystallography by Solvent Recrystallization

218 9.1 Solvent recrystallization allows for a more detailed optical crystallographic characterization of a 

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

220 9.2 Water recrystallization for inorganic oxidizers and salts

221 9.2.1 Add 1-3 particles or grains (approximately 1 mg or less) of the specimen to a small drop (5-7 mm in 

222 diameter) of distilled or deionized water on a microscope slide. It may be necessary to crush and disperse 

223 the sample with the tip of a glass rod. Alternatively, a portion of the specimen can be mixed with water 

224 and filtered to remove water insoluble components such as carbon, aluminum, and sulfur; a small drop of 

225 the filtered water extract is then placed on a microscope slide.

226 9.2.2 Examine the test drop using the PLM. Crystals typically form at the edge of the drop first. 

227 Distorted or subhedral crystals, if formed, can be pushed back into the center of the drop using a glass rod 

228 or metal probe to produce characteristic euhedral crystals. Multiple water-soluble components can be 

229 present in the sample and, depending on their concentration and solubility, can recrystallize at different 

230 times as the drop evaporates.

231 9.2.3 Examine the crystals using plane-polarized and crossed-polarized light. Note their crystal habit, 

232 relief, and whether they are isotropic or anisotropic. 

233 9.2.4 For anisotropic crystals, determine if extinction is parallel, symmetrical, or oblique. 
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234 9.2.5 Assess the interference colors to determine if the birefringence is low (less than 0.01), moderate 

235 (0.01-0.10), or high (0.10 and above). 

236 9.2.6 If observable, use interference figures to determine if the crystal is uniaxial or biaxial and the optic 

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

238 and, if the crystal is biaxial, estimate 2V.

239 9.2.7 Compare formed crystals to known controls or published references. Table 1 lists optical 

240 crystallographic properties for eleven common oxidizers. Document the crystals by description, sketch, 

241 or photomicrographs.

242 9.2.8 Dried crystals can be mounted in refractive index oils for refractive index determination, analyzed 

243 via microcrystal tests or other instrumental techniques (WK 67862). If the crystals cannot be further 

244 analyzed while on the glass slide, use a tungsten needle to transfer the crystals from the glass slide to 

245 another substrate, such as a potassium bromide pellet, for additional analysis. 

246 9.3 Organic solvent recrystallization for sulfur

247 9.3.1 Add 1-3 particles or grains (approximately 1 mg or less) of sample to a small drop (5-7 mm in 

248 diameter) of a suitable solvent such as toluene or methylene chloride. Sulfur, if present, will recrystallize 

249 as highly refractive, birefringent dipyramids (diamond-shaped) or form liquid drops that crystallize when 

250 touched with a tungsten needle.

251 9.3.2 Document the crystals by description, sketch, or photomicrograph. 

252 9.3.3 Compare formed crystals to known controls or published references. Table 1 lists optical 

253 crystallographic properties for sulfur. 

254 9.3.4 Dried crystals can be further analyzed by other analytical techniques such as Raman or SEM-EDS. 

255 If the crystals cannot be further analyzed while on the glass slide, use a tungsten needle to transfer 

256 crystals from the glass slide to another substrate, such as carbon tape, for additional analysis. 

257 10. Optical Crystallography Using Refractive Index Liquids

258 10.1 Organic and inorganic compounds present in explosives are characterized by their optical 

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

260 10.2 Mount 1-3 particles or grains (approximately 1 mg or less) of the specimen, or dried crystals 

261 obtained during recrystallization, as described in Section 9, in a refractive index oil. If previous 

262 examinations indicate a particular compound, mount in a refractive index oil to confirm one or more 

263 known refractive indices of that compound.

264 10.3 Examine using PLM to determine the optical crystallographic properties such as refractive indices, 

265 birefringence, extinction type (parallel, symmetrical or oblique), optic sign, and optic axial angle (2V). 

266 10.4 Refractive index may be determined using the Becke line method or dispersion staining.

267 10.5 Compare the optical crystallographic properties of the specimen to known reference materials or 

268 published references (6, 8, 12, 13, 22). Table 1 lists optical crystallographic properties for sulfur, ascorbic 

269 acid, sucrose and twelve common oxidizers. Table 2 lists optical crystallographic properties for four 

270 organic high explosives. 

271
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272 Table 1. Optical Crystallographic Properties of Common Fuels & Oxidizers

Compound Crystal 

System

Refractive 

Indices/

Optic 

Angle

Habit Comments

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(-)

NaClO3 (8) cubic 1.518 squares, 

rectangles

Ba(NO3)2 (8) cubic 1.571 cubes

octahedra

Sr(NO3)2 (6) cubic 1.586 octahedra

Pb(NO3)2 (8) cubic 1.781 octahedra

cubes

High relief in water

KClO4 (8) orthorhombic 1.4731

1.4737

1.4769

2V = 

50O(+)

rectangular 

prisms, rhombs

Low order interference colors

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 offcentered biaxial interference figure. 

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 

anhydrous. 

NaClO4.2H20 

(8) 

monoclinic diamonds, 

rhombs

Low relief in water. Some orientations 

exhibit oblique extinction. Refractive 

indices not known.

KClO3 (8) monoclinic 1.415

1.517

1.523,

diamond-shaped 

tablets, often 

truncated

Moderate order interference colors, 

symmetrical extinction
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2V = 28O(-)

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. Using a glass rod to move 

them to the middle of the drop causes the 

stable orthorhombic forms to appear.

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 = 169

NaNO3 (8) hexagonal 1.5874 (ω) 

1.3361 (ε)

rhombohedra High order interference colors and 

symmetrical extinction.

273

274 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.

MP = 204-206

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 = 81

PETN (13) tetragonal ω = 1.556 

ε = 1.551

MP = 141.5-142

275

276 11. Microcrystal tests

277 11.1 Microcrystal tests are chemical precipitation reactions where the target substance precipitates with 

278 the reagent. The crystals formed are examined using a polarized light microscope. Microcrystal tests are 

279 used in conjunction with other independent techniques to identify explosive compounds. 

280 11.2 Microcrystal tests are used to test for anions and cations of oxidizers and other substances, such as 

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

282 used in explosives analysis are listed in Table 3. 

283 11.3 Prior to casework, the reagents used for microcrystal tests shall be tested for reliability using the 

284 appropriate chemical standard and negative controls. Once it is determined that the reagents are 

285 producing the expected response, the reagents can be used for testing case specimens.
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286 11.4 There are four routine methods for conducting microcrystal tests (4). Test drops and reagent drops 

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

288 normally take place within one minute.

289 11.4.1 Method 1

290 11.4.1.1 Place a drop of deionized water on a clean microscope slide and add one or more particles of the 

291 sample with stirring, crushing and dispersing as needed. Keep this test drop small and from spreading on 

292 the slide.

293 11.4.1.2 A drop of reagent is placed near, but not touching, the test drop. Depending on the microcrystal 

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

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

296 reagent to the water droplet. 

297 11.4.1.3 Place the slide on the microscope stage.

298 11.4.1.4 Using a clean glass rod, draw one drop into the other drop and immediately examine the 

299 preparation microscopically using a 10X objective. Higher magnification may be used but care must be 

300 taken so the objective does not contact the test drop. 

301 11.4.1.7 Observe the entire preparation including the mixing zone and within each drop. Note the 

302 presence of characteristic microcrystals or the lack of a reaction. 

303 11.4.1.8 The formation of microcrystals corresponding to those obtained with the chemical standard is 

304 indicative for that anion or cation. 

305

306 11.4.2 Method 2

307 11.4.2.1 Follow 11.4.1 except add the reagent drop into the middle of the test drop.

308 11.4.2.2 Observe the preparation microscopically as described in Method 1. Note the presence of 

309 characteristic microcrystals or the lack of a reaction.

310 11.4.2.3 The formation of microcrystals corresponding to those obtained with the chemical standard is 

311 indicative for that anion or cation. 

312

313 11.4.3 Method 3

314 11.4.3.1 Follow 11.4.1 except add reagent particles into the middle of the test drop.

315 11.4.3.2 Observe the preparation microscopically as described in Method 1. Note the presence of 

316 characteristic microcrystals or the lack of a reaction.

317 11.4.3.3The formation of microcrystals corresponding to those obtained with the chemical standard is 

318 indicative for that anion or cation. 

319

320 11.4.4 Method 4

321 11.4.4.1. Place a small drop of deionized water on a clean microscope slide and add one or more particles 

322 of the sample with stirring, crushing, and dispersing as needed. Keep this test drop small and from 

323 spreading on slide.

324 11.4.4.2 Place a glass ring with an internal diameter of about 10-12 mm around the test drop so the drop is 

325 in the center of the ring.

326 11.4.4.3 Place a small drop of prepared reagent on a coverslip.

327 11.4.4.5 Place the slide on a microscope stage making sure there is sufficient working distance between 

328 the top of the glass ring and the objective.
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329 11.4.4.6 Add a small drop of dilute (approximately 10-30%) sodium hydroxide to the test drop inside the 

330 center of the ring.

331 11.4.4.7 Immediately place the coverslip onto the glass ring so the reagent drop is suspended in the center 

332 of the ring above the sample.

333 11.4.4.8 Observe the reagent drop for the presence of microcrystals or the lack of reaction.

334 11.4.4.9 The formation of microcrystals corresponding to those obtained with the chemical standard is 

335 indicative for that anion or cation. 

336

337 11.5 Table 3 lists twelve tests for anions and cations common to low explosives along with the reagent, 

338 literature reference, and method as described in 11.4. Microcrystal tests for additional ions can be found 

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

340

341 Table 3. Microcrystal Tests for Ions

Cation/anion tested Microcrystal test & reference Method

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

Method 4, if potassium present 

and ammonium ion absent 

method 4 test will be negative

Ammonium Chloroplatinic acid (4) 4

Sodium zinc/uranyl acetate (4) 4

Aluminum ammonium molybdate (4) 2

Magnesium Squaric acid (19, 20) 2

Barium Squaric acid (19, 20) 2

Strontium Squaric acid (19, 20) 2

Lead Potassium iodide (4) 1

Nitrate Nitron sulfate (1, 4) 1

Perchlorate Strychnine sulfate (4) 1, 3

Perchlorate Methylene Blue (7) 1

Chlorate Methylene Blue (7) 1

342

343 12. Fusion Methods

344 12.1 Fusion methods (10) are techniques that allow for the characterization of crystals by determining the 

345 melting point and observing optical crystallographic properties as the sample recrystallizes from the melt. 

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

347 inorganic explosives (9). 

348 12.2 The solid sample is placed on a microscope slide with a cover slip, or on a cover slip (according to 

349 manufacturer’s instructions), placed in the hot stage, and brought into the field of view. The temperature 

350 in the hot stage is raised as the sample is observed using the polarized light microscope. The melting 

351 point is documented and the temperature is lowered slowly while observing the recrystallization of the 

352 sample from the melt. 

353 12.3 Note the optical crystallographic properties such as morphology, crystal front shape, polymorphism, 

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

355 optic sign for each solid state as appropriate. 
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356 12.4 Most inorganic oxidizers found in low explosives have melting points above 300°C. These higher 

357 melting oxidizers are heated on a microscope slide using an alcohol lamp (9). An accurate melting point 

358 is not determined, but the sample is observed through the microscope as the slide cools to obtain various 

359 optical crystallographic properties, as described in 12.3. 

360 12.5 Compare the optical crystallographic properties of the specimen to published reference 

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

362 12.6 Mixed fusions 

363 12.6.1 If an explosive compound is indicated using optical crystallography, microcrystal tests, or other 

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

365 identity of the unknown (9, 10). 

366

367 12.6.2 A small amount of material from the specimen is placed near the edge of a coverslip, melted over 

368 an alcohol lamp, and allowed to flow under part of the coverslip. Limit the amount of specimen so that 

369 the entire area under the coverslip is not filled with the specimen. 

370 12.6.3 Once the unknown specimen has cooled and recrystallized, the known reference material is placed 

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

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

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

374 12.6.4 Using the PLM, view the zone of mixing as the preparation cools and recrystallizes. If the 

375 unknown compound recrystallizes into the reference material without differences in velocity, appearance, 

376 or the presence of a eutectic, this is strong evidence that the two compounds are the same.

377 12.6.5 Once cooled, the mixed fusion preparation can be inserted into the hot stage at a temperature 

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

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

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

381 the zone of mixing. Note: this can only be performed if the sample melts below the upper temperature 

382 range of the hot stage.

383 13. Documentation

384 13.1 All details necessary to support the interpretation made from the examinations are documented.

385 13.2 Examination notes are reviewed to ensure they reflect the particular characteristics used in the 

386 microscopical examination including measured, observed, or calculated values, descriptions, sketches, or 

387 photomicrographs.

388 13.3 Examination documentation is reviewed to ensure they contain sufficient detail to support the 

389 conclusions such that another qualified examiner could independently reach the same conclusion based on 

390 the notes or documentation.

391 14. References

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

393 Common Oxidizers,” The Microscope, Vol 55, No. 2, 2007, pp 55-58.
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394 2. Parker, R. G., Stephenson, M. O., McOwen, J. M., and Cherolis J. A., “Analysis of Explosives and 

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

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

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

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

399 New York, NY, 1940.

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

401 No. 1, 2000, p. 51.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

427 Applied Sciences, Westmont, IL, 2017.

428 19. Wills WF, Jr., “Squaric Acid Revisited,” The Microscope, Vol 38, 1990.
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429 20. Hollifield, J.M., “Characterization of Squaric Acid Precipitates,” The Microscope, V51 N2, 2003, pp 

430 81-103.

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

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

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

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

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

436 McCrone Research Institute, Chicago, IL, 1989.