Wood-grain decoration:
A process which becomes possible with vertical anodizing is a
differential anodizing process known as "Star-mite",
which gives a "wood-grain" pattern on the anodized extrusions.
This process involves a multistage anodizing treatment in which
the extrusions are first degreased and etched normally. They are
then AC anodized in an alkaline electrolyte, which often contains
a weak acid capable of barrier layer formation. Such an electrolyte
might, for example, contain sodium metaborate and boric acid,
and anodizing would be carried out at a current density of 2 A/dm2
for 15 minutes. Counter electrodes of stainless steel are used.
During this treatment a "wood-grain" pattern develops
on the extrusion surface. This is apparently caused by gassing
effects which influence the rate of anodizing in different areas,
so in order to produce the effect along the length of the extrusions
they must be processed vertically. Having produced the wood-grain
effect, the profiles are etched briefly in caustic soda, presumably
to remove the barrier layer of oxide, and then anodized conventionally
in a sulphuric acid electrolyte. The profiles are then usually
colored, either electrolytic ally or using conventional dyeing
methods prior to sealing. In Japan electrophoresis sealing is
used.
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