Thursday, October 12, 2006

Lead Shape

What shape is your pencil lead? I guess if you ask that today the answer would be ‘round’, with some implication of what a stupid question that was, what other answer could there be? (Apart from carpenters of course.)

Well leads weren’t always round. Early wooden pencils used square or rectangular leads, partly because when you started with a slab of raw graphite and cut it into sheets and strips you finished up with a square or rectangular lead. These days our modern leads are extruded from powdered graphite mixtures. Think of a tube of toothpaste, the graphite mixture is squeezed out (extruded) through a small hole. But the little hole could be any shape at all – round, square, triangular or anything.

So, why are leads round? There might be lots of manufacturing reasons for leads to be round, but from a draughtsman’s point of view, I can think of one major reason. If you hold your pencil perpendicular to the paper then the line it draws will be of constant thickness, no matter how the pencil is spun around, because the diameter of a circle is constant. Not so if your lead was square, rectangular or triangular. The thickness of the line would vary depending on whether the lead was orientated so that the outer edges of the line were drawn by the faces of the square or the corners of the square.

Engineering drawing places considerable emphasis on detail, including line thickness and darkness. It is important that a line is of constant thickness. The person at the front of the class with a big red pencil will deduct marks for variable line thickness and darkness! Draughtsmen are trained to hold their pencils perpendicular to the paper so that lines are of constant correct thickness. A round lead held at an angle to the paper starts to create a chisel point which will lead to variable line thickness if the lead is then rotated or held at a different angle.

Modern mechanical pencil mechanisms generally hold the lead quite tightly so it does not rotate around as you write, but this was not always the case. Not so long ago when many pencils were screw/slider mechanisms the lead was often only loosely held. As you wrote, a flat face chisel point was worn on the lead. Then if you rotated the pencil around a little to keep the point sharp the lead could rotate back as soon as you applied pressure, so you were back where you started.

The solution to this problem was to somehow stop the lead being able to twist around in the pencil mechanism. Here are two common methods from “older times”.

The first was to push your lead through a hole that it didn’t quite fit. Since pencil lead is not really that strong, you can fairly easily force it through. Here is an example from Eversharp where they make this part of their sales pitch. The round lead is forced through a sort of star shaped hole. I suppose back in the early/mid 1900’s the “rifled barrel” evoked powerful advertising concepts like military precision, and the ‘modern machine age’.

The second method was to not have a round lead in the first place. If your lead and the hole in the pencil tip were both square then you didn’t have any problem. Here is another Eversharp, this time with square leads! The little lead containers are stamped “Eversharp Red Top Square Leads” and on the lead chamber of the pencil is stamped “Use Only Eversharp Square Leads”. Actually the hole in the tip of the pencil isn’t all that square. Like the few others that I’ve looked at it appears to have been a circular hole and then pressed square. Sometimes they are more like a ‘circle squashed a bit square’ than a true square.

Well if I’ve had ‘The Lead Cup’, perhaps in the old days they had ‘The Lead Wars’? The “Squareheads” versus the “Rounders”? Here’s part of a Sheaffer advert from National Geographic magazine, May 1938. Note how they draw the circle around the outside of the square and claim over 20% increase in strength for round vs square leads. I guess the other guys just then drew a bigger square enclosing that circle, and claimed another 20% increase in strength! Luckily the “Lead Diameter Non-Proliferation Treaty” ended hostilities some time ago. (Yes I’ve gone mad again :)).


Shockbeton said...

Man, I love this blog.

I have a few curiosities to add to your very interesting lead shape discussion. The first relates to drafting pencils. In the days before the polymer-based micro thin leads were developed, there were attempts at achieving such micro-thin lines with ribbon shaped lead. One pencil of this type was the Faber Castell 9600.

100 years or so ago, Winsor & Newton made a refillable pencil and leads with a hexagonal section.

Lastly, a German company called Bleispitz recently (but now discontinued) made a carpenter style mechanical pencil with a triangular lead.


Kiwi-d said...

Hi Dennis
Thanks for these extra bits, very interesting, particularly the triangular lead. When I thought about triangular lead I just assumed equilateral, but their scalene triangle idea makes sense. Still, I wonder who the intended market was? I guess it didn’t work out seeing they stopped production. No disrespect to carpenters, but it seems a bit over the top for them. The traditional carpenters’ rectangular lead gives thick or thin line and that seems more than adequate from my limited experience. Actually I’ve never seen any of these mechanical type carpenters pencils in real life. Seen a few different varieties on the web, etc but none in real life. Guess I’ll have to make an effort to get one.
And I hadn’t previously noted the grade M on the hexagonal lead. I guess that’s medium, i.e. HB.

Anonymous said...

As an architect who has used a great number of different pencils and leads, I should advise you that while pens are held perpendicularly to the paper, pencils are often not. Indeed they are often dragged at an angle and are rotated to keep a consistant thickness (otherwise all lines would be the thickness of the lead diameter - and that would be a bad thing). A good draftsman can draw the finest to the thickest line with the same lead by changing angle. You will notice that any pencil designed for drafting has a circular barrel so that the pencil can be turned. They are free of so-called ergonomic grips.h

The second point is that lines are not always supposed to be consistant. A look at any hand drawn blueprint will show thickening at the ends of the lines or where they cross other lines. This can make the drawing more readable.

Kiwi-d said...

Hi Jack
Fair enough. I’ll certainly take your word for it when it comes to architectural drawing. However for mechanical engineering drawing I can only say that I was taught very differently. Variable line thickness was not acceptable, etc.

Steve said...

Hi Dave,

It's been more than 2 years since your last comment here but I have a question about my vintage eversharp pencil that I gotta ask so hopefully you'll see this. Story is I just bough a vintage eversharp not very dissimilar to the one in the picture above. The question is, I've run out of leads and I don't know what kind of leads (specifically, lead size/shape) to buy. I see square and round leads available on ebay, but I'm not sure if it'll fit my pencil. Any help regarding this?


Kiwi-d said...

Hi Steve. Well you will probably have to experiment. Square leads were the minority. Use a magnifying glass and look at the hole in the tip of your pencil. If it looks squarish, or like a circle that then got its sides squashed a bit squarish then you have your answer as to what lead it takes. If the whole is round then it will probably be 0.9mm or 1.18mm lead. You should find modern 0.9mm in a shop and thus be able to try it. probably have to snap it into shorter lengths.

Steve said...

Thanks DAVE!

KateGladstone said...

Mechanical pencils with square leads are VERY much alive and well ...