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I was reading the wikipedia page for pencil, and came across an interesting fact:

Prior to 1565 (some sources say as early as 1500), a large deposit of graphite was discovered on the approach to Grey Knotts from the hamlet of Seathwaite in Borrowdale parish, Cumbria, England. This particular deposit of graphite was extremely pure and solid, and it could easily be sawn into sticks. It remains the only large-scale deposit of graphite ever found in this solid form. Chemistry was in its infancy and the substance was thought to be a form of lead.

My question is, why was graphite thought to be a form of lead? The density of graphite is ~2.2g/cm^3, while lead is 11.34g/cm^3. They're both gray--but I have a hard time believing that color was the only reason people thought at the time that it was a form of lead. I'm sure I'm missing some context here that someone could elucidate.

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    Funny, what I find surprising is how people were ever able to recognize minerals in the first place. Have you even seen pure lead, let alone had a chance to measure its density? I don't think I have... and I'm not sure I would recognize it if I ever saw it. – user541686 Mar 9 '18 at 6:45
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    If every other property (they can observe) besides density is the same, than it can be easy for them to arrive to the conclusion that it's just a lighter variant of lead. The similarities are not only color. They have a similar softness, and leave similar markings when rubbed against paper, for example. – vsz Mar 9 '18 at 7:18
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    @vsz but a stick of graphite snaps, while a stick of lead bends (my grandparents had the original Cluedo set with a real lead bar for the lead pipe, so I've handled near-pure lead; anyway solder feels very similar) – Chris H Mar 9 '18 at 9:58
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    @Mehrdad Yes, I've seen and handled pure lead in high-school physics classes. But, really, I think the point is this: suppose that you, as a person who doesn't have much feel for what lead is like, came across a deposit of... something, in the ground. Would you say "Aha, this is lead!" (especially given that the one thing you surely do know about lead is that it's really heavy, whereas the stuff you found is pretty light) or would you go ask somebody who actually knows about minerals? – David Richerby Mar 9 '18 at 10:26
  • @ChrisH, Probably not pure. Lead, in most applications, is alloyed with antimony to improve its mechanical strength. – Solomon Slow Mar 15 '18 at 0:59
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Theses minerals were confused because they are quite similar in appearance, attributes and possible usage.

Graphite was previously called plumbago meaning the mineral Galena also called lead glance, which is a lead containing ore, not pure lead (plumbum). The density of pure lead glance (PbS) is only 7.60 g/cm3. They both look really similar and were indeed used for similar applications, for example namely in cosmetics. Kohl and mascara are charcoal – or mainly carbon – and just this galena respectively. Graphite was previously also quite often confused with molybdenite, (density: 10.28 g/cm3) a substance also capable of marking smooth surfaces.

Going by density alone is not very useful when classifying minerals, since densities vary, despite containing the most wanted metal. The numbers cited by you and me are for the pure compounds, usually not found as such in nature.

Galenit is also quite soft, clocking just 2-3 on the Mohs-scale, compared to 1–2 for graphite.

Galena: enter image description here
and graphite: enter image description here
molybdenite: enter image description here

All of these materials can be used as a solid lubricant as well. Making them of strategic importance for military purposes. England banned the export of pencils to Napoleonic France, since the containing graphite is ideal as a lining for casting cannonballs. (Source: Scientific American: Carbon Wonderland (2088))

Like galena graphite was also used to glaze or line pottery vessels, making them more fire proof.

The three minerals historically named galena, molybdena and plumbago have several common features — they are all soft, dark materials with a metallic lustre. Before the advent of modern chemical methods, these three substances were often mistaken for one another. Because galena (lead(ii) sulfide) was known to be a useful lead ore, it was commonly believed that molybdena and plumbago also contained lead. However, molybdena (which we call molybdenite today) was actually molybdenum(iv) sulfide, and plumbago was what we now call graphite.
(Anders Lennartson: "Made by molybdenum", Nature Chemistry, Vol 6, August 2014, p746.)

Most important is of course the actual application as a writing or drawing tool. Everyone now knows how graphite in pencils works, but a similar effect is achieved in using a stilus plumbeum. These drawing tools are said to originate in ancient Egypt and are allegdly also described by Pliny. This variation of metal point is today most often called a silver point, despite lead being the main component in most cases.

This then closes the circle since modern pencils, containing graphite, are in English often called lead pencil and in German always called Bleistift. Young students often wise crack about licking the tip of a pencil, being unsure whether "it is or is not dangerous" because "lead is toxic", "actually it is not lead and does not contain any lead, so it's safe". At least modern German word usage (and Danish, Dutch, possibly more) and many citizens still confuse the minerals.

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    Would this explain why pencils, which contain graphite, were - in my young day - referred to a "lead pencils"? They may still be called that. – WS2 Mar 8 '18 at 18:58
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    WS2: Oh. I just realized that Germans call pencils "Bleistift", i.e. "lead pencil". Thanks for the comment! – Eric Duminil Mar 8 '18 at 21:16
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    @WS2 I believe that's correct. This answer is interesting; I was never aware of any confusion between lead and graphite. My (incorrect) assumption was that people called them "lead" pencils because at some point in the past they contained lead instead of graphite, much like how some people call aluminum cans "tin" cans. – Justin Lardinois Mar 8 '18 at 23:53
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    Oh my god I just realised where the word "plumbing" comes from – Lightness Races with Monica Mar 9 '18 at 1:35
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    @WS2 I don't know if the term "lead pencil" is still common, but I'm not aware of any term other than "lead" or "pencil lead" being used to describe the stick of graphite down the middle of one. – David Richerby Mar 9 '18 at 10:35
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The wiki page on graphite contains a bit on it, ultimately this discovery came well before (200 years) graphite was considered something different than 'black lead' https://en.wikipedia.org/wiki/Graphite:

Historically, graphite was called black lead or plumbago.[7][29] Plumbago was commonly used in its massive mineral form. Both of these names arise from confusion with the similar-appearing lead ores, particularly galena. The Latin word for lead, plumbum, gave its name to the English term for this grey metallic-sheened mineral and even to the leadworts or plumbagos, plants with flowers that resemble this colour.

The term black lead usually refers to a powdered or processed graphite, matte black in color.

Abraham Gottlob Werner coined the name graphite ("writing stone") in 1789. He attempted to clear up the confusion between molybdena, plumbago and black lead after Carl Wilhelm Scheele in 1778 proved that there are at least three different minerals. Scheele's analysis showed that the chemical compounds molybdenum sulfide (molybdenite), lead(II) sulfide (galena) and graphite were three different soft black minerals.

It should be noted that it resembles lead ore, not lead...which would presumably weigh a bit less. Wonder if I have my old mineral test kit. Some properties one tests for in identifying a substance:

Hardness. Graphite and Lead are both 1.5 on Moh's hardness scale (https://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness). This meant not only did it have the same coloring, it had the same softness to it.

Streak. Both graphite and lead leave a streak on a streak plate.

Magnetism : Both lead and Graphite also show very similar behavior in the presence of a magnet, not magnetic but interacting with a magnetic field.

Graphite actually has a few properties that are quite misleading, as it is simply a carbon arrangement yet still acts as a 'semimetal' and operates as a semi-conductor.

Until you get into more advanced forms of chemical testing, lead and graphite behave much the same.

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  • Was there a scheme for classification of minerals around the time of the claimed discovery of the graphite deposit? Moh's didn't come for quite some time after. Approaching the question by thinking of different mineral tests today is good--but is the historical answer mostly about the color and softness? – spacetyper Mar 8 '18 at 16:47
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    @spacetyper - You are correct that the Moh's scale came significantly later as a standard classification, however alchemists have been doing variations of these tests for years prior (the hapless alchemist that tried to turn graphite into gold?). It doesn't take much to rub a sample against a rock to see if it leaves a streak or to indent a soft metal. Lack of standardization likely made it that much easier to confuse graphite and lead. – Twelfth Mar 8 '18 at 17:08
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    Thanks, I should have thought about the normal mineral identification techniques, and the fact that a common lead ore would not have similar properties to pure lead. – spacetyper Mar 8 '18 at 21:47
  • @spacetyper - just to add, magnets (at least naturally occurring ones) are available in this time frame. education.jlab.org/qa/historymag_01.html the similar magnetic behavior of lead and graphite might also have led them astray here as well. – Twelfth Mar 8 '18 at 22:01
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    Your last two sentences are inaccurate. Graphite is a conductor, not a semiconductor (though graphite was confused for lead long before electricity was understood). And it's trivial to distinguish lead from graphite by the fact that a block of lead weighs more than five times as much as the same sized block of graphite; lead melts but graphite burns. But, as you say earlier, it's harder to distinguish graphite from certain lead ores by primitive chemical means – David Richerby Mar 9 '18 at 10:45

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