Introduction to Microscopical Geo-Sourcing: Part 2 – Combustion Products
NOTE: Part 1 may be found here.
Preface by Richard E. Bisbing, McCrone Associates, Inc.
In the Locard tradition, forensic scientists most often use microscopic trace evidence as a means to connect victims and suspects with each other or the scene of the crime. The principal means to make that association is by direct comparison of microtraces on their clothing or objects at the crime scene with possible sources. For example, foreign fibers found on the victim’s clothing are compared directly with fibers comprising the suspect’s clothing, or paint chips on a screw driver are compared directly with paint from the window sill.
In a few cases, sourcing is attempted before suspects are identified and comparison samples obtained, most commonly in hit and run accidents where paint from the run-away vehicle remains at the scene of the accident. The paint is analyzed and compared with databases, often allowing the laboratory to predict the make, model and year of manufacture of the run-away vehicle. There have also been notable cases where carpet fibers were found on the victim’s clothing and, through cooperation with major car manufacturers, the carpet fibers were identified and determined to have been installed in a single model over just a couple of years. A model case using leaf litter appeared recently in the Journal of Forensic Sciences as a Letter to the Editor (Volume 48, Number 3, May 2003, page 696) [http://journalsip.astm.org/jofs/]. Although, sometimes painstakingly difficult and time consuming, these cases illustrate the potential value of microscopical geo-sourcing.
John Delly began a short series on the principles and practices of microscopical geo-sourcing, starting with minerals, in the last quarter and continues with combustion products here. We again invite our readers to contribute articles about other materials that can likewise be geo-sourced, such as pollen, seeds, sand, etc. As usual, we encourage the contributions to be accompanied with color images. We are not interested in news and views on the other geo-sourcing, that is, outsourcing IT services to some other part of the world. On the other hand, we would be interested in such projects as “Geo-Sourcing of Controlled Substances by High Field Deuterium NMR Spectroscopy and Stable Isotope Ratio Mass Spectrometry, “or “Geo-Sourcing of Cocaine Using Isotopic Ratio Mass Spectrometry (IRMS) and Trace Alkaloid Analysis” as a means for the U.S. Drug Enforcement Administration to determine the geographical origin of controlled substances, (http://www.emsl.pnl.gov/new/highlights/030100.shtml; http://www.carleton.ca/~bhollebo/Chem2303/AlkaloidAnalysis.htm)
Part 2 – Combustion Products
Combustion product particulates are microscopic dust particles that bear characteristic indicators that the original parent, or host, material was exposed to and/or altered by heat. The exposure to heat may be mild or extreme, and the host material may be almost anything. Many examples come to mind, such as the deliberate burning of coal or petroleum or gas or wood, or a combination of these, for the purpose of producing heat for personal comfort or survival in cold climates, and for cooking, or for a variety of industrial processes, including power generation. The burning may be accidental, as in forest and grass fires, or it may be the deliberate result of arson.
Most of us think of combustion resulting in char and ash, leaving no evidence of the original material behind. Actually, nothing could be farther from the truth. Combustion product particulates can provide valuable information about the original material and its origin. Physical and chemical changes that take place during the combustion process are numerous, but let us look at a typical, common example by way of illustration, the burning of coal.
What kind of products one obtains from the burning of coal depends upon the exact composition of the coal, the degree of coalification, and the efficiency of the combustion process. Let us start with the coal itself. Coal started as vegetation – trees mostly – which alters through pressure and heat of burial to the familiar product we all recognize as coal. However, the original trees are of different species mixes (i.e., many, many species in any one coal swamp) in different parts of the world where coal is found and their morphology is maintained; the soil where the trees grew is of different composition, and so the mineral content of the coal varies. The conversion of the trees to coal takes place over a long period of time; different deposits represent different degrees of carbonization, from peat, through various grades of bituminous coal, to anthracite. I remember driving through a very sparsely populated region of northern Scotland once – the kind of place that has only single lane roads and where you see only a few wild sheep – and was surprised at the unexpected sight of several men who appeared to be sawing the ground with long two-man saws. I stopped to investigate further. In the conversation that followed, I learned that this was a peat site. The ground itself consisted of very densely compacted, mixed vegetation, brown in color. It was fairly friable; nothing like coal; just highly dense, compacted, semi-rotted vegetation that still required a saw to remove, rather than a shovel. The men had commenced to terrace the land to make the sawing easier. By the use of handsaws only, the large blocks they sawed out were reduced to log-sized pieces. These peat logs would be sold in town for heating and cooking. Only naked-eye examination or a hand magnifier is needed to see the vegetable nature of the peat.
Underground, where the pressure and temperature increases, such peaty material becomes carbonized into one of the lower “ranks” of coal. Not friable now, the carbonized material becomes hard, and makes for a good fuel. Bituminous coal is widespread and commonly used for fuel in both domestic and industrial-type furnaces. Bituminous coal in hand-sized pieces is opaque and, although “black,” actually ranges through many shades of gray, blue, red-orange, and brown. If a piece of bituminous coal is sawed and polished, and viewed with a microscope using epi-illumination, the cellular structure of the original tree is plainly visible; except for anthracite, the coalification process does not destroy the original plant’s microscopic morphology – unless it is first silicified and/or not degraded by bacteria in an anaerobic environment. To truly study coal, sections need to be made in the cross (transverse), radial, and longitudinal (tangential) directions. This, incidentally, was the basis of a very famous trial in England over a hundred years ago involving the microscopist John Quekett, after whom the Quekett Microscopical Club is named: in England at that time, the government tax on the land was based, in part, on any mineral deposits; coal was regarded as a mineral. There was a dispute between the government’s microscopist and the landowner’s microscopist (Quekett) as to whether coal was present – the degree of coalification being the question. The case was decided in favor of the government. Quekett, nonetheless, was convinced he was right and, at his own expense, caused scores of sections to be made of the material, and distributed to microscopists worldwide so that they might see for themselves the microscopic characteristics that were the basis of Quekett’s position. Today, there are still small subgroups of industrial microscopists who study coal microscopically; they are called coal petrographers. When bituminous coal is ground down very thin, one can see that the material is not opaque, but, rather, beautiful shades of yellow, orange, and amber.
The Chicago Academy of Sciences’ former director, Dr. William Beecher, was planning a walk-through diorama of an ancient coal forest, and asked me for a photomicrograph of bituminous coal. Imagine my delight and surprise when the thin section revealed sections through the spores of perhaps Lepidodendron, one of the ancient arborescent (tree-like) lycopods. The orange colors of the spores are very attractive.
Anthracite is the final stage in the coalification process. It is opaque, very hard, and one of the best fuels; however, there is not much of it. There are relatively few known anthracite deposits, and the resulting coal is expensive and only used in certain processes where the expense can be justified.
The chemical analysis of coal is available from each coal-producing state in the United States, and from various geological offices in countries outside of the United States. The analyses vary widely in the presence and proportion of the various elements. Some coal deposits are very rich in sulfur. Because of environmental concerns, a potential user of high-sulfur coal has to budget for expensive “scrubbers” to reduce emissions of sulfur combustion products that lead to sulfuric acid (acid rain). But high-sulfur coal is cheaper than low-sulfur coal. Some countries do not have laws governing their emissions. The net result is that coal gets shipped throughout a country and between countries. It is important to keep in mind that each coal deposit has its own characteristic, chemical makeup determined by its original soil type, etc. The coal that is burned at any particular site may have originated hundreds or thousands of miles away. And coal-burning locomotives and ships are still common in many places in the world. Coal-carrying barges still transport coal across lakes and oceans.
A common way of burning coal in high-efficiency furnaces is to first pulverize the coal so as to increase its surface area. It is then fed into a roaring furnace either by itself, or mixed with petroleum. Let us consider a simple case first: suppose you scooped up a large shovel full of pulverized (powdered) bituminous coal, and threw it through the open door of a high-efficiency furnace which already had a roaring flame going. What is going to happen to the coal dust? Many things. Different things. The environment inside of a roaring furnace is very complex. There is hot air rushing about violently; there are different air current types near the furnace walls than in the center of the inferno; there are different air patterns nearer the grate than well above the grate. The draft causes a tremendously strong rush of hot air up the chimney stack. The heat is so intense that bits of the fire wall and grate are constantly being sluffed off. Into this, we throw a shovel-full of powdered coal. Some of the coal particles will immediately be caught in the updraft and sent out of the stack unburned, so these particles, when observed microscopically, will look exactly like unburned coal dust. They are comminuted, irregular in outline, and show sharp edges, angles, and fracture; very thin edges will be transparent to translucent and of an orange color. By top light and high magnifications, their cellular structure is usually apparent, and the particles can show high reflectivity. In a highly efficient furnace, not too many particles will be like this; most will fall back down because of their size and/or weight, or are caught on downdraft eddies, and are reintroduced into the flame.
Coal is a very complex substance consisting of many substances of different boiling points. The lowest boiling components are driven off first, followed by the higher and higher boiling point components. Of course, most of these low- to mid-boiling point products are within the coal particles, and the intense heat causes these internal components to burst out of the coal particle, leaving a jagged explosion hole where they made their violent exit. Some of these particles may get caught up in an updraft and sent out of the stack. Microscopically, these particles are now pock-marked. The surfaces are rougher and less reflective. When higher-boiling, tarry components are expelled, larger, gaping, explosive holes form throughout the particle, and not much is left now except mostly the mineral component. Because of the chemical composition variation even between particles, and with the different amounts of iron compounds present, this mineral component is variously colored. This is the ash stage. Domestic furnaces seldom produce particles beyond this type. The ash from large-size coal, say “Pocohontos Egg” (about fist-size) ranges from microscopic dust to ash pieces several inches across. The color varies from ash to ash, and within an ash: shades of grays, oranges, browns – many muted. The ash particles are porous and glassy. Microscopically, ashed coal at this stage appears just as it does macroscopically with large pieces: irregular, highly porous, dull particles with very rough, varied-color surfaces. Again, some of these particles invariably escape out of the stack.
If the ash particles suffer a longer, hotter time/temperature history, the ashy particles, which are essentially highly porous, roughened, mixed minerals, will start to round off; the thinnest edges of the lowest melting point round off first, then the next higher melting point mixture and so on. These partially rounded particles are called semi-fused, and the melted portions consist of mixed calcium, sodium, aluminum, iron, titanium, uranium, etc. …., silicates. Again, microscopic particles of this kind which leave the stack are identified as semi-fused by their now rounded areas. Holes, pock marks, and cavities are still evident, together with smoothly rounded areas. By top light, particles show areas of low to high reflectivity. The particles are usually quite colorful, with the colors being highly variable on the same particle and between particles.
If such semi-fused particles experience an even greater time/temperature history, the rounding continues until the melting point of the mixed silicates is reached, the particle becomes liquid, and complete rounding takes place. Larger, heavier such particles fall through the grate, along with the larger, heavier semi-fused ash. Microscopic particles which escape the stack resemble beads: round, glassy beads of different colors. Mass x-ray spectroscopy probing of these particles shows different compositions between coal from different coal deposits, and different composition from bead to bead. The beads often show inclusions of various kinds, usually entrapped gases or recrystallized minerals. Glass beads with entrapped gases microscopically will appear to have a black bead inside. They do not; the “black sphere” inside is actually hollow, and only appears black because of the large difference between the refractive indices of the entrapped gases and their silicate prisons. By scanning electron microscopy (SEM), some of the recrystallized minerals are quite attractive, especially when the outer round “skin” is broken and one can see the beautifully-formed crystals inside. Some spherical glass beads show beads within beads within beads, when beads already formed are incorporated into a silicate melt through collision. Bits of iron from the original soil or from the furnace walls, grate, etc., become incorporated into the silicate melt, and cause the beads to become different colors, in exactly the same manner in which borax bead tests and microcosmic salt bead tests are used to identify minerals in classic determinative mineralogy books. The iron and iron silicate particles both fuse, so that some of the beads are black and opaque and magnetic (magnetite spheres, Fe3O4), and some are deep ruby red (hematite, Fe2O3). Even some of these transparent red beads, and the occasional lightly colored bead must contain submicroscopic iron, because they too sometimes respond to a magnet moved above the specimen near the microscope objective, when the sample is in a liquid mountant. The tiny silicate bead is the final stage of the efficient combustion of a coal particle.
Given a general coal flyash sample, mount the sample in a liquid (e.g., nD=1.66), and view it by both transmitted and epi-illumination, and wave a magnet by the objective. Note the percentage of the sample that is rounded, if any. If rounded, how rounded – semi-fused? completely fused beads? just a few rounded? What proportion of the sample is opaque, translucent, and transparent? What proportion of the sample is magnetic? Are the magnetic particles rounded, partially fused, or both? Are there any unburned coal particles present? From the answers to these observations, one can come to a conclusion as to the efficiency of the process, and therefore the relative size and type of burner, or the absence thereof. Coal flyash particles are found in all industrialized areas, and areas of high urban population where power plants are present. Rural areas, equatorial areas, and semi-tropical areas can be expected to have less flyash in a settled dust sample. The flyash may be seasonal.
Watch for particle size/density. You can form a mental particle size histogram. Consider a highly efficient furnace of the type we used in the example. The stacks associated with these are often quite high. Heavier, larger particles will not make it out of the stack. Of the particles that do escape, the heavier, larger ones fall near the stack; the lighter, smaller ones travel farther away. From the visualized histogram, and some experience, the microscopist can form an estimate as to the distance from a source. Together with an air pollution engineer, this can actually be quantified. The engineers look at a stack’s height, and wind direction and velocity on a particular date. The microscopist supplies the particle size histogram and estimated density range. The fall-out of the particles follows Stoke’s Law, and with these data, the source can be verified. Or, working backward, the source can be located.
I worked on several cases like this, and from one, in particular, I learned another lesson. One city was suing another nearby city for fallout of garbage incinerator particles on its good citizens. In this case, I had the settled dust containing the garbage incinerator particles from the offended city, and what I needed was stack emission samples from the alleged source incinerator in the other city. I notified the engineers who were going to do the meteorological workup, stack height, etc., of my needs. They later supplied me with five samples, as the accused city had not one, but five identical incinerators operating simultaneously. I was delightfully surprised to see that, although all five incinerators were of the same manufacture, there were characteristic particle peculiarities about each of the five incinerators – much like five examples of the same make automobile and each of which has its own peculiarities. There were differences in the amount of air, draft, internal air flow, etc., in the five incinerators, which affected the degree of burning. During the trial before a judge, I was not only able to convince him of the offending city’s responsibility, I was also able to tell him which of the incinerators accounted for most of the fallout. The lesson here is that, if you have reference samples for direct microscopical comparison, you may be able to pinpoint a source. This technique of incinerator-sourcing was successfully applied in both Florida and Connecticut.
Now would be a good time to consult The Particle Atlas, and call up the descriptions and photomicrographs of coal and coal flyash. The Atlas includes examples of all of the particle types discussed above. Look at chain-grate stoker and compare this with the series of coal flyashes. Try to assess the degree of efficiency and the probable size of the combustion source from the morphology shown by the particles in the photomicrographs before reading the accompanying description. There are additional study photomicrographs in The Identification of Atmospheric Dust by Hartley and Jarvis, Central Electricity Generating Board, London (1963).
Petroleum is an oily, flammable, bituminous liquid that, in its raw state, varies from almost colorless to black. It occurs in many places in the upper strata of the earth. It is a complex mixture of hydrocarbons with small amounts of mineral matter and, rarely, pollen and spores. It is prepared for different uses by various refining processes that result in gasoline, naphtha and many weight and composition grades of heating fuel and diesel fuel for operating machinery.
As with the combustion of coal, the efficiency of the burning varies widely, depending on whether combustion takes place in a small domestic oil-burning furnace, or in a large power plant. The fuel can be quite sooty – sometimes deliberately, as when generating carbon black for use in rubber tires, or black paint pigment. Heavy construction and earth-moving equipment produce thick, sooty clouds of black smoke. Homes burning oil are more efficient, but not as efficient as power plants that depend on optimizing and maximizing the combustion process to completeness.
If we collect some of the visibly sooty combustion product from heavy earth-moving equipment, and view it with a light microscope, we will see “black snowballs;” the spherical particles are not smoothly spherical, but “soft” “sooty” spheres with almost granular outline. If we now place a dissecting needle over these particles and depress the coverglass above them while watching them, what we see will depend on exactly how efficient the combustion was. If the combustion is incomplete and inefficient, we will see the spherical particle “squoosh” like a pancake, and stay “squooshed,” and a droplet of oil may be seen to be squeezed out. Or, if tarry, no oil will be seen coming out, but the flattened particle stays flattened, and may smear on moving the coverglass.
If the oil is being burned in the near absence of air for the express purpose of generating carbon particles, then what we see will be black “snowballs” consisting of agglomerates of zillions of tiny individual submicroscopic carbon particles. When the coverglass is depressed on top of one of these, the zillions of particles that are loosely held together in spherical form will immediately break apart into its zillions of individual grains. It takes very few of these particles to render a slide preparation too dense to be of further service. A little pigment goes a long way here.
In heating applications, the oil will be sprayed into the furnace. As with the coal combustion, sometimes raw fuel is expelled from the chimney, but most is burned. On being sprayed, the oil takes on a spherical shape. Lowest boiling materials are driven off first; then next higher; and so on. As the droplet becomes thicker and thicker, the fraction boiling out will be driven from the droplet interior through the “skin” formed on the surface by the process taking place minutely sooner at the surface, and the surface will become tarry/semi-fused with numerous pockmarks and blowholes. On continued time/temperature exposure, even the tarry material is driven off, until the particle is mostly glassy mineral; if large, these are usually hollow. Microscopically, they appear like orangey-brown glass Christmas-tree ornaments. The color may be uniform, or what used to be blowholes are repaired with molten silicates, but the repair is thinner over the blowhole sites than at the edges so that the particle takes on a fenestrated appearance, sort of like a port hole, with generally rounded “panes” surrounded by darker brown framing. When the coverglass is depressed on these particles, being glassy hollow spheres, they break, usually just into two, three, or a few pieces, or sometimes many, but always curved fragments, again just like a Christmas tree ornament would break.
On still further heating to the melting point of the glassy sphere, the hollow sphere collapses and coalesces into a smaller solid glassy bead consisting of mixed silicates; the last remaining mineral component of the original droplet. These are, on average, much smaller than the glassy beads from coal combustion.
Perhaps surprisingly, many of the sooty black spheres, the intermediate black spheres, and even some of the final spheres respond to a magnet. The iron responsible for this comes both from the soil mineral component of the original oil, and from the pipelines and furnace components deteriorating.
If analyzed by x-ray spectrometry, vanadium is usually detected in more than normally expected percentages. Vanadium is traceable to the cracking catalyst, and is a good indicator of a processed material.
The term “blowing one’s stack” comes from the practice of admitting high pressure air up the stacks for the purpose of clearing out accumulated carbonaceous build-up from the stack’s inner walls. Ships usually blow their stacks when out at sea. There are regulations against blowing their stacks within certain miles from ports; however, there are frequent incidences of petroleum-burning ships blowing their stacks while in port, but during the nighttime. Here the resulting dirtying is a great nuisance, and the problem is to trace a particular sooty fallout to a particular stack.
Companies that produce carbon black are frequently in trouble with the nearest residential area. The owner of a newly-painted white house may claim damages when the paint becomes dirty. I worked on one case like this in which a company complained about another for being responsible for the general dirtying of its newly erected building. On examination, I found the “dirtying” to be due entirely to a fungus; the new building was built along a river in a wooded area where conditions were favorable for such mold growth. The building was repainted with an antifungicide in the paint. In another case, a carbon-producing plant was being blamed for a recent fallout. The company hired me to look at the fallout and compare it with their product. They were identical. Investigation showed a problem on the night shift which resulted in an unreported release of the company’s product.
Again, the efficiency can be estimated from the degree of conversion of the petroleum oil from the spherical, oil fuel, through the sooty carbon stage, through the non-sooty black sphere stage, to the semi-fused and glassy sphere stage. Incidentally, by top light, the loosely-held sooty spheres are of very low reflectivity; the non-sooty, semi-oily particles have a “wet” look, sort of blue-black with specular highlights; the glassy spheres show high reflection with images of the light source. Again, call up all petroleum entries in the combustion products section of The Particle Atlas.
According to a recent Peoples Energy Corporation publication, the prevailing scientific theory is that natural gas was formed in nature millions of years ago when plants and tiny sea animals were buried by sand and rock. As additional layers of mud, sand, rock, plant, and animal matter continued to build up, the pressure and heat from the earth turned dead plants and animals into petroleum and natural gas. Even gaseous fuels have a minor mineral content, and mercaptans (sulfur-containing compounds) are added to the methane to give it an unpleasant smell for leak detection (natural gas is tasteless, odorless, and colorless). Complete combustion results in extremely tiny spheres; incomplete combustion of natural gas results in carbon being produced. I recently switched from a wood-burning to a gas-burning fireplace. In the gas-burning fireplace, the methane flows up through a bed of sand from a manifold of pipes with holes in them. Where there is plenty of air mixed with the gas, the flame is blue, and combustion is fairly complete, but there are relatively few places like this. In most places, the flame is yellow by design; it is a more pleasing “warmer” color, but the result is incomplete combustion, so that carbon is deposited at many places along the ceramic logs. Recently, I learned of a complaint from a homeowner who had had white carpeting installed, and was now attempting to hold the building contractor responsible for replacing the carpeting. One of the sources of combustion in this homeowner’s living room was an unvented gas-burning fireplace which was in frequent use. Air currents in the room made for differential distribution of the carbon particles, and uneven dirtying.
Channel black is another carbon product produced in the near absence of air. Look up channel black in The Particle Atlas.
Before natural gas was discovered, gas was manufactured using coal and petroleum-based substances. The process created by-products, such as coke and coal-tar. Coke was sold for residential heating and industrial uses. Look up some of the coke” entries in The Particle Atlas. Coal tar is used in asphalt paving, roofing materials, and consumer products.
Other Combustion Products
When I was still burning wood in my fireplace, I was burning two species of oak exclusively. The resulting light gray ash was periodically sent down into the ash pit. When this oak-burning ash is viewed microscopically, the evidence of cellular structure is still present. Black carbonized fragments (charcoal) are also present. On crossing the polarizers, the whole field-of-view lights up because of the high content of calcium oxalate crystals in the cells of the wood. This crystalline component can be found in tobacco ash as well. Look at an unburned cigar leaf to see the well-formed calcium oxalate within the cells. Cigarette ash also is quite dramatic between crossed polars, but for an additional reason. In the case of cigarette ash, the highly birefringent particles are from precipitated carbonates in the paper wrapper of the cigarette. This carbonate-laden paper is responsible for the maintenance of the ash on the end of the cigarette, without which the end would be constantly falling off. The veinous structure of the tobacco leaf is maintained during the burning process, and remains to be seen as carbonized branching veins in cigarette, cigar, and pipe ash.
The characteristic plant cellular structure seen in wood-burning fireplaces and in tobacco combustion ash can also be seen in the burning of all plant materials. Thus, bagasse still shows its original plant structure after being burned. It is the plant waste product of sugar cane, after the sugar has been extracted. Burning is a common way of disposing of the waste. Forest fires, prairie fires, crop residue fires, and grass fires all leave carbonized residue that bears the cellular structure of the parent plant.
Sometimes the burning is slight, as in scorched cotton fabrics, or cotton or paper fibers entrapped in a pharmaceutical vial preparation which has been autoclaved. In these cases, the identifying morphology is still totally present, only the cells are shades of pale yellows, oranges, and browns by ordinary transmitted light. Rather than burned, these particles are perhaps best described as “thermally degraded;” they have experienced only the initial stages of carbonization; just enough to impart a bit of browning to the vegetable fibers, and perhaps some brittleness.
Now look up the combustion products for wood, bagasse, esparto grass, and cigarette ash in The Particle Atlas and read the description, preferably while looking at known slide preparations of each material. Look up fireplace dust, and don’t miss the surprising makeup of commercial charcoal briquette ash.
Think of a camp fire at night. Somebody throws on another log, and sparks fly up in the heated gases. If the particles of combustion are collected, you will see that the inefficiency of a simple camp fire results in unburned fragments, partially burned fragments, semi-fused fragments in many cases of really hot fires, and the mixed mineral content from adhering soil, and intracellular oxalates.
The latest fuel recently announced is called “biodiesel.” It consists of french fry and other waste grease, animal fats, cooking greases, and soybean oil. A mixture of these wastes produces a biodiesel fuel that is now being tested in buses. About 25 million gallons of leftover waste oils are produced each year. About 1.5 million gallons of the fuel were sold during a recent 12-month period for nationwide testing. It will be interesting to discover what the chemical composition will be like, and what the microscope will reveal as to the morphology of the combustion product.
Lately, there has been an increase in residential soot contamination traced to the burning of aromatic and other kinds of candles; gas chromatography with mass spectrometry (GC/MS) and large-volume injection detects the unburned paraffin in the soot particles.
Once samples of known coal flyash, petroleum flyash, wood burning, trash burning, etc. are looked at, in association with the foregoing discussion, it would be a good idea to look at general settled dust samples from urban areas in industrialized countries, and compare these with rural areas. Then compare samples from northern and southern hemispheres during various seasons. Finally, given an unknown sample, concentrating on combustion products alone, speculate as to the degree of industrialization, the variety of combustion sources, the relative distance to such sources, the relative efficiency of the sources, and what this implies regarding probable furnace size and possible industrial uses. Of course, combustion products are only one particle type. We have already considered mineral particles and what they mean in Part 1 of this series. Now combine the two particle types, and the amount of information one can obtain as to geographical origin and human activity increases. When other particle types are now added to these, the information that can be obtained about the origin of the sample becomes astounding.