Louise, Regarding your request for on coloring anodized
aluminum, I suggest you contact John S. Brana in San Francisco. His
website is http://www.ganoksin.com/gnkurl/ep808q
Here are two paragraphs that John wrote for my new book "Gold,
Platinum, Palladium, Silver & Other Jewelry Metals" on page 103. He
compares anodizing aluminum to anodizing niobiumand titanium.
"Anodizingniobium and titanium is a much easier process than with
aluminum. All you need to anodize is increase voltage/time in the
electrolytic bath to build up oxide layers to form various colors.
Anodizing reactive metals such as niobium and titanium is a
high-voltage, low-amperage process performed with nontoxic
electrolytes. So in practice, you can achieve a rainbow effect on the
same piece of jewelry just by varying the rate at which you pull the
item out of the electrolytic bath. " In contrast, anodizing aluminum
is more labor and time intensive. Aluminum needs to be degreased,
deoxidize, anodized, dyed, and sealed. After degreasing and
de-oxidizing, the raw aluminum must have its pores opened, usually
through a sulfuric acid bath in order for a thick oxide layer to
form, so that the aluminum will accept the dye. Once the dye is
impregnatedinto the honeycomb-shaped pores through a heat bath, the
color is sealed by boiling the aluminum in hot water or with a
sealer. Although you are leftwith one even colored object, an endless
color palate can be achieved through mixing dyes. However, you cannot
produce rainbow effects, as with Niobium."