John Davis wrote:
>
>Great stuff! Can you give any indication of how much currect is needed (per
>square inch of object?).
Glad you find it interesting and potentially useful.
>
>Also, have you ever tried masking some areas of the film ( hour numbers)
>before the dying process, so that they come out a contrasting (or clear)
>colour?
>
See my recent posting on re-anodising through etched 'holes' which would allow
red/blue/green etc. lines on a yellow or gold background. The companies who
specialise in dyes for anodising produce some wonderful metallics in every
shade from pale gold to deep copper but they only supply 1kg minimum
quantities. Try to make personal contacts in firms which specialise in
anodising and they may sell smaller quantities. Does anyone know of suppliers
of smaller quantities?
If there was sufficient interest I might consider buying 1 kg of e.g. gold dye
and selling it on in 50 gram lots although I would have to charge appropriately
to make this worth the fair amount of trouble involved. Or perhaps the BSS
might consider buying and re-selling for a fair profit? Fifteen years ago
Sandoz charged about £UK25 per kg so it will be a lot more than that now.
Household dyes work quite well but often lack the depth of colour of the
commercial product.
I have a voltage/area/time graph compiled by a casual research student which is
reliable for small workpieces. I suspect the flattening out of the curve is
due in part to our power supplies 'running out of legs' so I hope the
commercial anodisers out there don't laugh too loud.
GIF on request.
To illustrate the durability of the anodic film I kept small samples in my
labcoat pocket for years alongside bunches of keys, coins, small drills, broken
cutters, bits of oilstone slip and half-eaten sandwiches and they remained as
good as the day they were made.
SAFETY While both myself and Mike Shaw quite rightly drew everyone's attention
to possible hazards this is a safe and easy process if handled with ordinary
common sense. Dozens of students in my care produced hundreds of anodised
examples with not a single cause for concern except for a technician who didn't
bother to switch on the ventilation over extended sessions and temporarily lost
his sense of smell.
THEORY for them as wants more!
Why does the film build up in a honeycomb of micro tubes? My readings of 20
years ago (can't recall the source) suggested the following reason:
The aluminium sheet will always have a thin coat of atmospheric oxidation.
When connected to a positive supply and placed in an acid electrolyte small
holes will be eaten through this oxide film to produce 'pits' of positive
charge. This takes place all over the plate and, because all the pits have a
positive charge, they become equally spaced by mutual repulsion eventually
settling down into a close packed 'honeycomb' arrangement.
No doubt ideas have advanced since I read that but it did seem to make sense.
Rainbow sundials here we come!
Tony Moss.
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