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The shimmering oil slick effect is produced after applying black ceramic paint and then firing them at 1000 degrees. After that a lustre is applied followed by another firing at 750 degrees.
The canisters stand at around 17 cm tall by 11.5 cm in width but as they are hand made you'll never get two EXACTLY the same - which is part of their charm I think.
A rubber seal comes with the lid ( which should be removed when washing) to improve the airtightness of the seal and so extend the longevity of the contents.
This shop -WWCeramicsUK has a range of items made in a similar style which would match these canisters if you wanted to extend your collection.
More Unusual Tea Coffee Sugar Canister Ideas HERE