hi everyone this video is an introduction in the glass evidence for forensic science let's start by looking at case a803852 police responding to a noise complaint discover a car with fractured glass and you can see an image of it here it appears to be caused by two bullet impacts broken glass is found both outside of the vehicle and inside of it police ultimately find and detain a suspect and on that suspect's shoe they find glass fragments which you can see pictured here as a forensic investigator based off of this information can you determine the type of glass that was found on the suspect can you figure out if that matches the windshield that had these two bullet impacts the order that the shots were taken was it left and right or right and left the direction or angle of the shots and the speed of the shot my hope is at the end of this video you can so why investigate glass as a forensic investigator well glass is everywhere you'll find it on store fronts you'll find it making up a good chunk of the top of the car and homes are laced with glass with windows and doors because of this glass is most likely going to be broken with any kind of criminal activity that occurs storefronts anything that may encounter that cars unfortunately are often broken into and breaking the windows are one of the most common ways of doing that and if there's any kind of home entry or skirmish odds are some glass will be broken glass is typically seen as being class evidence glass is frequently mass produced for things like bottles or car windshields so discovering it it can be matched to any group of individuals that happen to own that same bottle or car however glass can be seen as individual evidence if you are trying to reassemble glass fragments back into the object they broke off from if there is a potential match all the pieces fit together that can conclusively show that some fragments originated from a specific item so what is glass well glass is a hard brittle material that is a amorphous solid what we mean by an amorphous solid is that atoms are randomly arranged and vibrate in random ways this causes a pretty uneven molecular structure which results in glass having a tremendous diversity of fracture patterns that's great for forensic investigations anytime there's diversity or unique characteristics we can use that to determine the origins of the material we're analyzing glass is produced by melting in high temperatures silicon deoxide lime or calcium oxide and sodium oxide there are variations in this mixture but most glasses contain at least two of these three and glass occurs in nature anywhere where there is extremely high temperature such as a volcanic eruption or a lightning strike and sand when those two combine you can form glass such as this obsidian glass you can see here which is very common to see around active volcanoes the compounds used and the methods used for producing glass will actually change its characteristics how it behaves and how it appears so it's important for us to know the most common varieties or types of glass should we encounter them in the field one very common type is called soda line glass this is what makes up most bottles and windows it is the most common type of glass makes up up to about 90 percent of manufactured glass it's mostly silicon dioxide and sodium dioxide and it is recyclable giving more reason as to why it's so common this is different than lead or crystal glass this is found in fine art pieces and in fine glass like fine dining scents what makes this unique is that 18 to 40 percent of the mixture is made out of lead oxide it is much more dense than a soda lime glass and less brittle and then there's pyrex or kimax glass this glass is designed for cooking and lab glassware because with the addition of boron in the glass mix it's able to be resistant to high temperatures if you've ever done chemistry use something like a hot plate you know that the glassware you use in a science lab is very very resilient to high temperatures it typically won't shatter and that's because that addition of boron to the mixture making it a pyrex glass now all of these glasses can be colored and the way we color glass is by adding some kind of a metal oxide for example yellow glass is made by adding nickel oxide and blue glass is made by adding cobalt oxide just a little more information that could help you identify and match glass that you may discover in the field should it have a color let's go through the major ways of manufacturing glass one of the more common types of glass that you'll come across is something called an annealed glass this is glass that is produced by allowing the glass to slowly cool if you've ever done ceramics and you put your ceramic in a kiln and it slowly lowers the temperature it's the same idea here but with glass instead of using a material like clay this kind of glass is weak and very easy to break and it shatters into a regular sharp fragments this is the kind of glass where if it breaks you want to stay away and use gloves to handle it and observe you've encountered it because it's used when safety isn't too much of a concern we're not dealing with high temperatures or protecting the surface and it's very cost effective it makes up things like the outside of a picture frame this is different than laminated glass laminated glass is created when two or more panes of glass are fused together with a plastic middle layer this is designed to resist breaking this kind of glass is a lot thicker and a lot sturdier and used in applications where we really can't have that glass break into pieces that could release whatever's inside that's why we see it with things like car windshields aquariums and doors when laminated glass breaks those broken chunks will actually stay adhered or stuck to that inner plastic layer so we'll typically see those fragments stay within the frame that's why when you have the shattering of a car window like you can see in these pictures the glass remains on the car it doesn't typically scatter in every direction this is important if you're driving and something impacts your windshield the last thing you want is those fragments to enter the car or get on to people who may be inside of it another type of glass is tempered glass tempered glass is subjugated to extreme temperatures or chemicals by doing this we make some interesting forces on the glass itself this treatment by high temper chemicals causes the outer surface to compress will be pushed in from both sides and it makes the inner surface be under tension meaning that it's being pulled in opposite directions as you can see in that diagram that causes two forces to be against each other the inner force and the outer force as a result of that tempered glass has a unique property of breaking into small granular chunks instead of long jagged and random edges this is great if you need a glass that if broken the shards that come from that break will not harm any individuals that come in contact with it which is why this is a very common glass for things like shower doors glass tables cookware things where humans are readily interacting with the glass and if it breaks we want to make sure it doesn't cause any harm next up is bullet resistant glass often called bulletproof glass bullet-resistant glass is laminated and tempered glass and is often created with multiple layers so we'll have tempered glass with plastic between another layer tempered glass plastic another layer of template glass and so on this produces hard and soft layers within the glass which gives it flexibility and ability to resist something that's coming at it with tremendous force like a bullet the soft layer adds flexibility so as the energy that bullet goes through the glass the glass can flex ever so slightly to disperse that energy and it keeps those fragments attached so that they don't break off in either direction the thicker the glass the more protection you'll receive from bullet resistant glass and you can actually have glass that's so thick that it can flatten the bullets that the bullet can't penetrate most of the layers in the bulletproof glass so as a forensic investigator you're going to be presented broken pieces of glass and you're going to need to be able to determine what kind of glass it is is it soda lime is it pyrex something you'd find in a door is it something i'd find in a shower door thankfully each type of glass because of its unique composition and treatment produces a unique result when treated with a variety of scientific tests those the most common tests are a density test to see what the density of the glass is a refractive index test just look for the presence of becky lines underneath the microscope and to analyze the fracture patterns that may be produced in the glass let's look at each test in turn let's start with density every type of glass has a density that's specific to it you can see in this data table bottle glass has a density of around 2.5 grams per milliliter whereas tempered glass is higher at 2.98 to find the density of glass we just use the density equation which is density equals mass over volume gram over milliliter is the most common measurement so let's try it out let's say you're handed a piece of glass and asked to find its density first thing you want to do is find a scale and measure its mass for this example let's say the mass is 5.1 grams next you're going to want to use water displacement to find its volume the way this works is you place an object in water that's been measured to a known volume so here my graduated cylinder is starting at six milliliters place in your glass and then you measure how much the water is displaced or moved the difference in that measurement so six minus eight is going to give you the volume of the object that is submerged so for our example we have two milliliters six minus eight now we wanna use the density equation so density is mass over volume so i have 5.1 grams divided by 2 milliliters in this example which gives me a density of 2.55 thankfully we have reference tables that will show us what the average density is for most unique types of glass with this result of 2.55 that puts this glass closer to a window glass now let's think about refractive index notice in this picture i have a strong side of a glass but it appears as if the straw gets cut and then begins over to the right something isn't right when i visually try to look at that straw that's because of refraction that occurs with light when light travels from one medium to another let's say going through the air to glass like you can see in this image it changes direction that's what we mean by refraction you can see the light bending upward and then out this is why the straw appears bent the light that's hitting your eye is being refracted because that light in the air changes direction refracts when it enters the water in the glass let's look at an example of this in more detail so here you can see a light striking a piece of glass when the light hits a surface we can actually measure the angle at which it hits and we refer to that as the angle of incidence notice there's been a horizontal line drawn here we can draw a parallel line to the point that it hits and we're going to call this the normal line why draw all of this well by making that normal line and knowing the angle we can do some mathematic calculations which are very insightful if we look deeper into what's happening when this light hits the glass notice how the light bends towards the normal line this is typical when you're dealing with the refraction at whatever angle it ends in if you draw the normal line the light will bend towards that normal line notices also that the light line goes from being solid to dash and if we were to see it go out it would then become solid what's that indicating to you is that speed of light actually changes based on medium or the substance that it's traveling through let's look at that in more detail we know that light travels at different speeds in different substances because of this we can measure light speed for every substance you could possibly encounter in the field and use that unique figure to determine what kind of substance we're working with this is what we mean by calculating a refractive index it's the speed of light in the vacuum divided by the speed of light through a particular substance this equation works because the speed of light in a vacuum is one nothing that we know of is faster than speed of light in a vacuum there's nothing disturbing light's ability to travel because of that any other substance i measure is going to have a refractory index greater than one you can see here for different kinds of glass heat resistant glass is about 1.47 whereas lead or crystal is 1.56 so we can conduct this test using a laser pointer or fixed light to see what the refractory index is for our class to be able to answer the question what kind of glass am i analyzing now it's not always the most practical thing however to be measuring the speed of light that can be costly and complicated so it's more common for forensic investigators to use this immersion method to figure out the refractive index of the piece of glass here's how it works you set up a series of test tubes that contain liquids of known increasing refractory index so you can see here in this image on the far left with one i have a liquid that's a refractory index of 1.33 and then i increase that index for each subsequent tube we would then put a piece of our glass that we're examining into each of these tubes and notice that as we get greater here it becomes more and more challenging to see the glass to the point where the glass almost appears to disappear whatever measurement that is wherever that glass begins to disappear the reason it's disappearing is because the refractory index of the liquid is identical to that of the glass so by using this method we can rapidly determine what is the refractory index of any specimen we're examining set up your test tubes in increasing or magnitude put in your samples and see which one disappears another way of examining glass that's been submerged is to look for becky lines if the refractory index of a liquid and the glass are the same this is what you would see looking at it under a microscope when it submerged noveculi you just have glass that appears to disappear in the liquid but you can end up with the result like this notice that there's a line of light inside the glass fragment this happens if the glass has a higher refractive index than that of the liquid it's in that line of light is what we mean by a becky line it's a short halo of light inside or outside a submerged piece of glass now when the glass is a higher refractory index we see that light on the inside if glass has a lower or to say it another way the liquid has a higher refractory index we'll actually see the becky light on the outside of the glass fragment another way to help you calibrate your submersion to see what liquid refractory index you need to make it disappear is if you see a becky line inside okay that means the glass is higher so i gotta have a higher liquid or the inverse if the light is on the outside now let's talk about fracture patterns when a glass is fractured so let's imagine a ball dropping on this glass pane it's first going to form radial fractures or outside lines notice in this gif on the left you can see that as the ball slowly collides into the glass there it goes you can see straight lines first form after that secondarily fractures will form in concentric circles known as consensual concentric fractures so we start with straight radial fractures and then after that concentric factors start to circle around where the radial lines have formed why does this happen and why does it happen in this order well let's look at this diagram if i start with a piece of unbroken glass and i impact it from the left what's going to happen is i'm going to develop tension on the opposite side of the glass which will break out and then after that i have tension points above and below the surface that was impacted that first tension break on the back is going to be where we have our radial fracture and then on the front surface with those secondary attention breaks is where we'll secondarily have the concentric fractures form so if you are presented a piece of glass that's been fractured like this could you determine which side of the impact using only radial and concentric fractures the impact occurred was it on the top or was it on the back well you can a reminder that radial happens first and it happens on the opposite side of the impact so if i see where those radial fractures are formed i know the opposite side of the glass is where that impact initially occurred or i can look at the concentric factor fractures those circular fractures are going to occur only on the same side as the impact here i have two different fracture patterns impact one and impact two do you think you could figure out just by looking at it which impact occurred at a higher speed and could you explain why well the one on the left occurred at a higher speed than the one on the right how do i know this well at higher speeds you have tighter concentric fractures you can see how tight those circles are in impact one and how close they are to the impact point whereas at a slower speed you're going to have much more spacing between your concentric fractures now when you're dealing with some kind of ballistic or bullet fracture there are some unique patterns that are helpful in forensic investigations when a bullet hits glass it actually takes a cone-shaped piece of glass out and it goes with it the direction the bullet is traveling you can see that here in these images because of that the entry hole and the exit hole are going to be of different size the entry hole will always be smaller than the exit hole in a pane of glass because that cone shape of material is being pushed out and that is a greater breakage than the initial breakage from the bullet enters just like you can see in these images here we can use this to determine the direction of an impact or which side that impact was made on the pane of glass now let's say three shots are fired into a pane of glass could we determine the order in which they're fired we can and the trick is those radial fractures those fractures that happen first when fractures occur first they will stop any other fractures that occur in that direction the new fracture can't run through the old fracture like you can see in this image here i know the impact on the left happened before the impact on the right because it is stopping that radial fracture from continuing so knowing this take a moment look at the diagram on the top right do you think you could determine the order these shots a b and c were fired in well looking at it i'm pretty confident that shot c happened first if you look at the portions that are highlighted you can see that the radial fracture from shot a is being blocked on the left hand side and on the right hand side the radial fracture from shot b is being blocked by the fractures in c okay so which shot happened second for that i want to look at the interaction between a and b from that you can see that the fractures in a are preventing the fractures from b to grow you can see that right there where they meet so that gives me confidence that first we had shot c then shot a and we finished with shot b shot b is b radial fractures are being blocked by both of those shots could i determine the angle that a bullet was fired at if i was presented a piece of evidence like this absolutely a reminder of what happens when there's a severe or high speed impact in glass you'll actually have some glass fragments go forward yes bulletproof glass doesn't really cause that to happen but let's say we're working with another type of glass those glass fragments that move forwards towards the shooter or towards the direction of the impact are called back scatter those fragments will give us an idea of the direction of the initial shot we'll also have fragments that come out the back of wherever the impact has occurred for those fragments that fall out on the other side of the glass they can be very insightful on angle if i am striking perpendicular to the glass i should see fragments moving in the exact same direction but let's say i'm aiming a little bit off to the right if i'm aiming at the right that's going to cause fragments to cascade out through the left and opposite is true if i have a projectile that strikes from the left that's going to cause fragments on the other end to jump out to the right so this will give us a lot of insights into what angle that shot occurred glass can also be fractured by heat glass that's exposed to high temperatures can produce what we call a heat fracture and you can see some examples of that hair here heat fractures are unique to the other fractures and that they create regular wavy patterns through the glass and it's always on the side that's exposed to the heat if you see any kind of fractures like this heat is the only thing that can produce it glass can also be easily scratched and you probably have encountered this with your smartphone at some point since glass can be easily scratched those scratches can leave patterns which are very helpful in forensic investigations for example a car i very likely will be able to find very fine scratches that show me where the windshield wipers have been scratching on the side window of a car let's say something is stuck in the mechanic mechanism that rolls a window up and down that's going to cause some unique scratch patterns that can indicate to me that there's some kind of object there that i should dig deeper maybe crack open the car door to find so how should you collect and document glass evidence that you encounter in the field well first you want to identify photograph and label glass samples before they're moved from where you find them then you want to collect the largest fragments you can find think about tests like submersion you want to be able to have as many fragments as possible you want to always wear gloves glass is sharp then you want to identify the outside and inside surfaces of a piece of broken glass if i find a fragment on the floor can i pinpoint okay what will be the inside of say a home if it's a home window and what would be the outside for that fragment and then you want to note any trace evidence that's found on the glass if there's blood or any kind of soil we want to document that and make sure that that evidence is properly analyzed so let's go back to the case we started with case 8 8 a 8 a803852 when police responded and you saw these two fracture patterns and you found those glass fragments on the suspect could you now determine what type of glass that is on the suspect if it matches what i'd find in a windshield the order the shots happened the direction or angle and its speed my hope is that you can thank you and i'll see you next