G'day Chris here, and welcome back to Clickspring.
I've just finished doing most of the formation
work for these clock hands.
and after giving them a quick rub on this
India stone,
I'm about to start the process to get the
final blue finish.
Generally its presented as 4 steps - Harden the part,
Heat temper it to Blue, Polish the surfaces,
and then Heat again for a cosmetic blue.
But first things first, what is it about heating
steel that makes it turn blue?
I think the first thing to mention is that
it's not some sort of surface color
like a dye or pigment, but rather a fascinating
by product of the wave nature of light.
Heating steel causes a very thin oxide layer,
to build up on the surface, and importantly,
the thickness of this oxide is directly proportional
to the temperature of the metal.
When full spectrum light strikes the surface,
some is directly reflected by that oxide,
and some passes through and is reflected by
the underlying metal,
and so travels a tiny bit further. The two
waveforms are now slightly out of phase,
and interact as they leave the upper surface.
Depending on the thickness of the film, some
colors are cancelled,
and what we see are those colors that remain.
When the film is thin, we see a pale straw color.
As the film gets thicker other colors are
removed, and color we see
gradually shifts from brown through purple
to a rich blue.
This is known as thin film interference; its
what causes the rainbow of colors in soap bubbles,
and a very similar effect generates those
fantastic iridescent colors
you can see on some birds and insects.
In our case if we get everything right, the
result is a magical deep blue color on our steel part,
that no other process can quite match.
And I should also mention that the result
is not just a pretty blue color,
but also a very reliable proxy for determining
temperature,
which we use during the tempering process
in a moment.
So with a bit of the science behind the process in hand,
we can make a few statements about what factors will affect
the quality of the final blue color we're pursuing .
For one thing it seems reasonable to expect that a uniform oxide thickness
would give a uniform color, so it follows that we'll want to heat the part as uniformly as we can.
I've made a dedicated set of bluing tools,
to help me do just that.
The second thing we can say is that the surface
finish of the metal
will have a big effect on the light reflected
from the metal surface.
Broadly speaking, the better the polish, the
better the blue color.
And thirdly, cleanliness is crucial.
Any sort of contamination will not only affect
the way the oxide forms,
but also the light reflection,
so we want the work to be meticulously clean
just before we start heating.
So lets get started
Hardened steel will take a mirror polish much
more readily than a soft steel.
So the first step is to harden the work.
In this case, because I'm working with such
thin parts,
I really would like to avoid any oxidation
pitting at all.
You will have seen me use soft iron binding
wire to wrap most parts,
but in this case, I've made a custom tray
to hold them in.
I'm still using boric acid to coat them, but
it forms a much more reliable glaze
over the top of the parts with this tray,
and completely protects them from oxidation.
The tray and contents are brought to a red heat using a propane torch and then quenched in oil.
Once the quench is complete, that boric acid coating comes off very easily with a little boiling water.
Now at the moment, the parts are glass hard,
and much too fragile
to be safely handled without risking breaking
them,
so I need to temper that hardness.
And just to prove the point, here's wider
scrap of the same steel prior to hardening,
and here it is immediately after hardening
.
You can see it doesn't take much force to
shatter the work into several pieces.
Tempering reduces this brittleness, and gives
me a little more leeway to handle the parts,
while at the same time making the steel easier
to polish.
As it happens the temperature we want to temper
the work to,
corresponds to the blue oxide color, a little
over 300 degrees celcius,
which is why you see the steel blued twice
- once is for this tempering,
and the second time at the very end, is just
for the appearance.
And you can see with this hour hand, just
how sensitive
the bluing process is to a temperature differential.
I inadvertently overheated the middle of the
tray as I was setting up for the shot,
which made one end of the part change color
much more rapidly than the other.
I start to regain control over it by swapping
it end for end,
but a quick burst of heat from above was really
the only way to get it back on track.
Now that's ok for the temper bluing, but its
much too coarse a response for the cosmetic bluing,
and definitely something I try to avoid doing
if I can.
The second time around, for the minute hand,
I was paying much closer attention,
and directed the heat more centrally around
where the part was resting in the tray.
You can see the result is a much more consistent
and uniform color change,
along the entire length of the part.
OK, so the hands have been tempered from full
hard to blue, and are ready to be polished.
And there are many different ways to go about
this.
I'm going to use abrasive paper for most the
initial work,
but I could just as easily have chosen to
use abrasive stones,
or perhaps diamond pastes on polishers instead.
The main thing I'm keeping in mind, is to
try to maintain the crisp edges,
and to avoid rounding the flat surfaces.
When working on the broader surfaces, I'm
trying to keep the movements reasonably short,
with an even pressure.
The final surface finish comes from using diamantine powder on a tin lap.
Again with short moves to maintain the flat
surfaces.
At this point most of the hard work has been
done; but as a last step before bluing,
I'm using this fine abrasive stone to break
the edges of the work,
and to give the parts a little more character.
Ok, so I'm about to start the second bluing,
the one that is just for appearance.
So the parts need to be thoroughly cleaned.
This light solvent that I'm using is essentially
a lighter fluid,
and removes the bulk of the oil and particles
left on the surface from the previous operations.
I finish the cleaning using some rodico, which
is a sort of soft, tacky putty a bit like bluetac.
It pulls the last of the solvent residue from
the surface, and leaves the parts quite clean.
And you'll notice that I don't touch the parts
with bare hands from here on,
until the bluing is finished, to maintain
that clean surface.
The bluing tray is pre-heated as before, as
evenly as I can get it,
and then the parts are placed into the middle
of the tray one by one.
And this is the moment of truth,
where I find out how well I've performed all
of those previous steps.
If the color has any sort of flaw,
like for example you can see that the very
tip of this hour hand doesn't quite go fully blue,
then the part must be taken back to the previous
polishing stage.
To remove the oxide, re-clean, and then have
another try until its right.
The color I'm aiming for is that rich deep
blue, with just barest hint of purple;
but even with all of this effort to make the
process controlled,
that color can shoot by before you know it,
and again if that happens,
its back to polishing stage, for another try.
There are also several other factors that
can affect the outcome too,
beyond what I've mentioned here, including
the composition of the steel,
the ambient conditions, as well as the nature
of the light being used to illuminating the part.
All of this can be experimented with to further
improve the final result.
Thanks for watching, I'll see you later.
If this is your first Clickspring video welcome.
You've just been watching a very small part
of a much longer term clock making project,
so be sure to go and check out the rest of
the videos from that series.
If you're looking for some new projects for
your lathe or mill,
then take a moment to visit clickspringprojects.com
where you'll find a range of plans available
for download,
including plans for some of the tools I've
made to help me construct this clock.
And finally if you'd like to help with the
creation of these videos,
then have a look at the Clickspring Patreon
page.
Thanks again for watching, I'll catch you
on the next video.
