What Causes This Striping On Pennies?

These striations/strata are "mottle effects" from an improper or inadequate mixing of copper, tin, and/or zinc when smelting and planchet production takes place(as with my Indian below). However, I am uncertain of the exact composition of your Aussie Penny, but I would bet it is also made from copper and tin or zinc. I really dig these examples and I hope this answers you question. I could elaborate further if you wish.


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Thanks Sap for the elaboration...well stated. This brings up another question. Has anyone ever witnessed this phenomenon in modern Cu coinage? Copper is not my cup of tea, but I can't ever remember encountering a modern with mottling. Perhaps current minting methods and QC prevents this from occurring and/or they are pulled before leaving the Mint.

I can think of three possible reasons why you wouldn't see the effect as often in modern copper coinage:

- better quality control, as Benji said.

- they're not as old, so the different oxidation rates haven't had time to mature yet. Let 'em ripen for another decade or two and the streaks will appear.

- don't forget that much modern "copper" coinage is merely copper-plated; if the copper plating is normally electrically applied, this should create a smooth even film of metal, without clumps of inconsistency on the surface.

people commonly refer to these as lamination flaws.
There can be two types of parallel lines of different colour on copper coinage - the one mentioned above by Sap and Benji, and another where the surface layer has 'peeled' and the exposed surface has become toned on exposure to the air i.e the blackness observed on that halfpenny. The peeling effect can be caused by carbon powder not being correctly absorbed into the alloy mix and then being spread in a thin layer under the surface when the sheets are rolled prior to the planchets being punched out. The carbon absorbs moisture, expands and lifts thin layers from the surface of the coin which then turn a different colour.They are generally in parallel areas across the coin because the rolling of the sheets spreads the carbon
The most spectacular of the streaky planchets occurs where you get 'blond' or white sections on the coins where the alloy wasn't mixed correctly.
I will dig a few coins out to illustrate

This is a 1941 Australian penny and is a good example of the alloy not mixing correctly. The surface of the coin is smooth - ie no lamination flaws.
The light coloured areas are not the result of cleaning or damage, its the actual coin colouring as scanned