Date: Thu, 16 Dec 1999 12:41:00 -0700

Author: "Michael Thomason"

Subject: RE: Large Chalkboard Erasers WARNING - LOOOONG


We have 27 square meters of blackboard real estate on each side of each of
our two two-sided physics lecture hall rotating stages. I think I have
developed close to the ultimate blackboard cleaning system. I am still
working on my intelligent blackboard cleaning robot, but until then, this
will do.


Essentially, use professional window cleaning tools with plain water.
These tools are far faster, more thorough and easy to use than sponges,
towels, or flat mops. Blackboards are enough like glass that it turns out
these tools work best. Lets not even talk about erasers - felt or foam.
And we are not talking about the squeegee you clean your car windshield
with. (Those *won't* work.)

You can buy these tools here
or here
*but preferably at your local professional cleaning supplies shop, monster
hardware store, or maybe your school*. These tools are available locally in
any college town for a few dollars a year without fabricating any special

There are three useful tools: the washer, the squeegee, and the 18-inch
bucket. The squeegee is not for cleaning the board but for drying it. The
washer and squeegee are "T" shaped tools available in lengths up to three
feet. I have tested all the lengths and observed what the student employees
who clean these boards every hour migrate to.


18-inches to 22-inches long works best for the washer. Washers have a
"plush" synthetic fabric sleeve that holds more water than sponges and has
the perfect surface for wiping chalk off boards. You can hold the washer by
the handle of its "T" or add a pole any length you like. The sleeve
detaches from its frame and is replaceable for a few dollars when it wears
bare after a few months use.

We hold the washer under running water in the sink on either side of the
rotating stage to rinse it out and soak it before each use - they never get
or need washing. The washer should be dripping wet to wash the chalk off
the boards. The running water is also important to get the chalk out of the
washer. No matter what you clean with, eraser, sponge or window washer,
once the cleaner surface becomes saturated with adsorbed chalk, it will just
smear the chalk on the board rather than removing it, until the chalk is
removed from the cleaner.


The washer gets the chalk off the board but leaves it wet. If you need it
dry during the break between classes, *then* hit it with a three-foot wide
squeegee. One wipe is all it takes. The three-foot wide size works best
for the squeegee. It is amazing how fast a board dries once you use one of
these professional squeegees on it. It won't dry as fast for you people in
soggy climates as it dries in our 20% humidity, but it will dry fast enough.

Using the washer *and* squeegee is faster than an eraser and vastly improves
the visibility of notes on the board. Erasers just spread out the chalk,
reducing the contrast between the color of the board and notes written in
chalk. An advantage of the two-sided rotating lecture hall stages is that
we can wipe the blackboards on the side of the stage that is back in my
lecture demonstration laboratory and leave it wet, giving it an hour to dry
before it is rotated back out into the lecture hall.


The last tool is the 18-inch wide bucket, specifically designed for these
window washers. You can pop the washer into this bucket to drain and leave
the bucket on the floor next to the stage or sink or mount it on a wall.
For longer washers I use trays designed for long paint rollers.

High-quality slate and slate-surface chalkboard manufacturers will tell you
water should never touch the surface of their chalkboards; you should clean
them dry with those chamois-backed foam erasers. That just isn't practical
in a high-use environment like ours and it doesn't provide the high-contrast
visibility of getting all the chalk off with water. I have decided to just
replace the boards if they wear out (which hasn't happened in twenty years

Just think of your blackboard as a *window* into physics.


The window washer and squeegee will take every bit of chalk and water off
the blackboards, but a lot of the chalk and water will drip or run onto the
floor. Now you have to mop the floor! The puddles are a slipping hazard
for instructors and they leave thick layers of chalk on the floor when they
dry. I keep a mop adjacent to each stage but instructors have accepted this
tradeoff and never even mention it. The custodians mop the floor once/night
or so.


Instructors are mobbed by students asking questions the moment they stop
lecturing. This is an important component of the students' learning
experience (probably much more valuable than the lectures). Instructors
attempt to schedule office hours immediately after their lectures to
capitalize on students' interest while the lecture is fresh. Any energy an
instructor puts into cleaning blackboards immediately after class is lost
student interest. Many students who would otherwise wait their turn will
leave when they see their instructor erasing the material they were just

Our lecture halls have student projectionists with wages paid by the
"Classroom and Media Engineering Support" office (not Physics). They
weren't excited when I asked them twenty years ago to start cleaning the
blackboards in-between lectures every hour. I frequently remind their
supervisor "Blackboards are an audiovisual medium and in physics lectures
they are the most important medium."

If a lecturer uses only blackboards and wireless microphone, no
projectionist is provided, so I make sure video and computers are used often
enough to keep projectionists around to clean the blackboards. A side
benefit is the projectionists count as employees I partially supervise and I
don't even contribute to their salaries. I provide a valuable service to
physics students and instructors by freeing them to think about physics (or
maybe how to drop the course) after lecture rather than about erasing the

Michael Thomason, Director, Physics Learning Laboratories
University of Colorado, Boulder, Department of Physics

In the matter of physics the first lessons should contain nothing but
what is experimental and interesting to see. A pretty experiment is in
itself often more valuable than twenty formulae extracted from our
- Albert Einstein
A Moszkowski, Conversations with Einstein, page 67, Horizon Press (1970)

-----Original Message-----
[]On Behalf Of Richard Berg
Sent: Friday, December 10, 1999 10:25 AM
Subject: Re: Large Chalkboard Erasers

We have a gizmo consisting of a 3-foot long aluminum frame wide enough to
accommodate a series of about six sponges (about three feet). It is kept
in a water trough which keeps it wet and contains a shelf for squeezing it
out. Using this you can clean a three-foot board very nicely with a
couple of swipes. It takes less thn five minutes to clean eight sections
of blackboard three feet tall and ten feet wide, going back and forth over
each one.

I can put a drawing on the web if anyone is interested.

We do this ONLY in the lecture halls - have been since before I came here
in 1972. It really does cut down on the chalk dust. Slick as a cat's
back in mating season.

Dick Berg