"Conan The Librarian" wrote in message
...
Wolfgang wrote:

I still make my own glass knives for microtomy.......um, but the process
is
a bit more technologically sophisticated than "knapping".

I'd be interested in hearing more about how you do this. The closest
I've ever come to that would be using a piece of broken glass as a wood
scraper. :-)

Experience with broken windows, bottles, etc., suggests that glass breaks
randomly. Not so. In fact, breakage is highly predictable....and
controllable.....under certain conditions. Various machines have been built
for a long time to control breakage in order to leave a sharp, straight edge
on pieces of glass of the right shape and size for use in microtomes. The
one pictured he

http://www.canemco.com/catalog/histo...nifemakers.htm

is fairly typical and looks much like the one I use. In the following
description it will help to know what the raw material looks like:

http://www.2spi.com/catalog/knives/histo.shtml

A strip of glass is placed on the stage of the knife maker so that it
extends left to right against the two flat pronglike stops at the base of
the head mechanism. The left edge is aligned with a mark on the left stop
(it helps if you save the picture and blow it up....but it's still not real
clear). Turning the crank on the right side of the machine causes the head
assembly to lower and press the glass down onto the stage via a couple of
rubber pads and an square steel anvil (the front edge of which is visible as
a bright white vertical stripe at the front end of the head mechanism) that
hold it securely. The glass is actually raised above the stage proper by
maybe a couple thousands of an inch by two parallel flat ridges visible as
ribbon-like structures on either side of the slot running down the center of
the stage. The slot contains a wheel type glass cutter that is activated by
the large knob on the front of the machine. The knob is pulled out and then
the button on top of it is held down, thus raising the wheel against the
glass, and the knob is pushed back in, scoring the glass. Now, the crank on
top is turned clockwise.....VERY.....slowly. This increases the pressure of
the anvil which is pressing down directly above the scored line. Since the
glass is suspended along two lines and the downward pressure is exerted
evenly between them, the glass MUST break cleanly along the scored
line......in theory.

In fact, it takes a lot of practice to make a clean, straight break. The
key to the whole process is stress relief.....and this is where the VERY
slow pressure increase comes into play. Glass is actually a fluid. The
verticle stained glass panes in the medieval cathedrals in Europe are
measurably thicker at the bottom because the glass has flowed (ever so
slowly) downward over the centuries. By increasing the pressure on the
anvil very slowly, you give the stresses time to dissipate to some extent
through the molecular structure of the glass. If you turn the crank slowly
enough and pause frequently (we're talking something on the order of five
minutes elapsed time here.....if done properly) you can actually watch the
fracture line slowly creep up through the thickness of the glass.

At this point you raise the head by turning the main crank on the side of
the machine (the top crank is never reversed.....it is used only the
increase the pressure) and you have the remainder of the strip on the right
and one perfect square of glass on the left.....if you used the correct
alignment mark. There are several marks, for different widths of strips.

Reinsert the square under the head with one of the corners pointed toward
the back of the machine until it stops against a pair of metal tabs
(unfortunately pretty much invisible in the photo). The small dark
structure at the front end of the slot on the top surface of the stage is a
slide which, when pushed forward, rises, exposing another pair of prongs
that capture the opposite corner of the square. Imagine a diagonal line
running through two opposite corners of the square. Now, turn the line on
its center by a couple of degrees. The glass square is held in a position
such that when it is held down as before and scored, the score line is
canted off the corners by those couple of degrees. Thus, when the square is
broken in half you are left with two roughly triangular pieces. In fact,
they aren't quite triangular....there is a small step at one corner. One of
the angles formed in the last break is slightly more than 45 degrees and the
other is slightly less. The angle opposite the one with the step in it is
the new business end of the knife. The edge here was created from a
theoretically perfectly straight piece of glass and will be (also
theoretically) straight and perfectly horizontal when the knife is stood up
on its flat edge.

Once again, practice doesn't quite match theory. For one thing, the
molecular structure of glass causes what is known as a conchoidal fracture
pattern....it just doesn't want to break in straight lines. Additionally,
perfection isn't necessary for most operations. The operator, knowing this,
doesn't increase the pressure on the anvil as slowly as he might and, rather
than watching the break line creep slowly through the glass he allows his
bored gaze to wander and listens for a distinctly audible pop.

If everything is done just right, the result is two perfect glass knives.
What actually happens most of the time is that I increase the pressure on
the anvil relatively quickly, make a dozen or so knives, discard ten of
them, and end up with two that will do well enough in less time than it
would take to make the two really good ones. This is really a more
efficient use of resources because the glass is a lot cheaper than my time
and the glass knives are only used for rough work....down to half a micron
thickness....anyway. For ultrathin sections....60-100 nanometers.....we
always use a diamond knife.

But I do think there's a real dichotomy there.
Only if you ignore the bulk of the spectrum in concentrating on the
infrared
and ultraviolet cranks at either end.

Well, we are on ROFF, aren't we? :-)

Yep, that's us, mostly monochromatic and largely invisible to most of the
world.

...dull the plane irons.....so I could
sharpen them again.

Ack. Not me. I sharpen my tools on a setup that consists of multiple
granite surface plates with wet/dry sandpaper of various grits affixed to
them (also known as the "Scary Sharp" method of sharpening). Until I
dedicated an area of my shop to that, I would go as long as I could
between touchups (and I often paid the price for it). Now that I don't
have to drag out my sharpening supplies, I tend to hone my plane irons on
a regular basis, but I still don't like it.

I used a couple of silicon carbide stones that I had made by a local
manufacturer....400 and 600 grit, if memory serves. I tried various jigs
and fixtures but never liked any of them. But I loved to work those blades
on the stones.....actually very similar to using the planes. I always
finished them with tripoli compound on a leather strop. Polishing the
blades makes a HUGE difference!

It interferes with the whole zen thing of making wispy shavings.
(Speaking of zen, ask Claspy about his one-sided shavings.)

Um.....yeah.....moebius later.....remind me.

I'm currently interested in making and flying kites, making cheese, and
making paper. Would you believe it, in a city the size of Milwaukee I
cannot find a decent retailer for materials for any of those activities!


You have got to be kidding me. I don't know about kites and
papermaking, but *cheese*?! In Wisconsin?!!


Chuck Vance (the mind boggles)

Ainna?!

Wolfgang