Fellow Shadow Watchers
Continuing my occasional series of practical approaches for metal sundials here
is a distillation of my personal notes on anodising aluminium prepared over
many years of practical experience. Aluminium has a normally dull and soft
surface but anodising can transform this into beautiful multi colours with a
weatherproof surface hard enough to resist all but the sharpest instruments.
Interested? Then read on!
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Anodising Aluminium in the Home Workshop
1. You will need a large acid-proof plastic container. Any polyethylene
bucket or tank will do into which your workpiece will fit with a three inch gap
all round. Alternatively I made purpose built tanks out of marine plywood
covered with a liquid-proof layer of glass-fibre resin and surfacing tissue.
After years of use these tanks are still in good working order.
Just one very large tank will serve but of course this will need a lot of
electrolyte.
2. You must have good ventilation as the fine bubbles of gas which rise to the
surface produce an weak acidic mist which is barely detectable - and then one
day you discover you have lost your sense of smell! There is also the possible
risk of male impotence which is why this was once known as 'etcher's disease'.
You have been warned! VENTILATE!
I have three healthy sons but that was before I began anodising!
3. You can anodise with low voltage AC or DC but if you use AC the Cathode¹
mentioned below MUST be of lead not aluminium.
4 The cathode (negative connection) can be formed from a sheet of aluminium
bent to fit around the interior walls of your tank as a loose liner¹. Leave a
projection tab to bend over the edge of the tank with a terminal to which the
negative wire is connected if you are using DC . If your power supply is AC
then the cathode must be sheet lead.
5. Part-fill the tank with electrolye made by adding one part of concentrated
sulphuric acid to nine parts of water. Wearing goggles and gloves pour the
acid SLOWLY down the side of the tank into the water. NOT the other way
around. Pause if too much heat is generated.
6. Experiment with a piece of aluminium plate about 3² square to get the idea.
Drill a hole in one corner and attach a pure aluminium wire by clinching with
pliers to ensure a sound contact. 1/16² aluminium gas welding rods are perfect
for this job.
If contact with the wire is loose then the anodic film may form in the joint
and this will stop the process. Design items to allow appropriate wire
attachment - leave a cut-off 'tab' or drill a hole in the base into which a
filed taper can be wrung/seized¹ tightly.
Hollow items can be held on an expanding zig-zag made to spring open into the
inside. etc. etc.
7. Holding the object by the wire dip it into a strong solution of caustic
soda for a few seconds until it fizzes to remove natural oxides and grease.
Rinse in clean water.
8. Suspend in the anodising tank supported from some kind of busbar. I use a
square brass bar over the tank drilled at intervals with screws to clamp the
wire into the holes and ensure a good contact.
9. Connect the positive lead of your power supply to the workpiece via the
busbar and the negative lead to the Cathode lining of the tank.
10. Set the voltage to about 16v and in a few minutes fine bubbles of oxygen
should rise from the workpiece. The oxygen produced by the electrolysis of the
water in the electrolyte combines with the aluminium at the surface to produce
crystal-clear aluminium oxide (grindstones are made from it!).
It is thought taht the oxide is produced is microscopic tubes¹ rising from the
surface rather like a honeycomb. Electrical contact is maintained through the
tubes and new oxide continues to form at the base thickening the film.
After about 20/30 minutes you can switch off the current, remove and rinse in
CLEAN water. The aluminium will have a whitish coating but DO NOT TOUCH IT or
you finger prints will be captured for ever!
If the object is now immersed in a cool dye solution - there are special dyes
made for this but most household dyes work fairly well - the dye molecules will
fill the fine glassy tubes to colour the item. Simply boiling in clean water
for a few minutes will then cause the outer ends of the tubes to swell and seal
off leaving you with a brightly coloured object which has a soft velvety feel
and is impervious to general wear and weather.
To get metallic golds and bronzes you will have to enquire of companies such as
SANDOZ who may part with samples. They usually supply in expensive 1kg tubs
which contain enough for 1000 years of home use.
Voltage and Current
Very small items such as rings and jewellery need a low voltage for a long time
whereas larger items need a higher voltage.
I have a GIF of a voltage/current/time graph derived from experiment which you
can request when you get started but by setting 16 volts and allowing the
current to take care of itself will be a good beginning from which to build up
your own experience.
Multi-colour anodising, photo-printing or writing on the anodic film with felt
tip pens etc. are all areas in which I achieved good results and are there for
you to experiment.
Try it! It's easier than it sounds but do wear appropriate protective gloves
and eyewear etc. when handling acids and don't forget to ventilate well.
Have fun!
Tony Moss
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\ ** ******
\\ ** **
\\ ** *******
*\\ ** **
*\\ ******* *******
**\\
***\\ Tony Moss, Lindisfarne Sundials
*****\\ 43, Windsor Gardens, Bedlington,
*******\\ Northumberland, England, NE22 5SY,
**********\\ 55° 07' 45" N 1° 35' 38" W
Tel/FAX +1670 823232
Mobile: 07970 208 540
Website: http://www.lindisun.demon.co.uk
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