Question on mint frost

Longacre

I should know the answer to this, but what causes mint frost? I assume that the coining process itself, when the dies squeeze the polished planchet, cause the devices to appear frosty. Is this correct? Exactly how does this happen? Does the pressure cause the flow of the metal to appear frosty? Thanks.

numobri

If I'm not wrong,the frost is added to the dies,as the dies are used,the frost goes away.

Frosty coins are the first out of the dies.

MrEureka

Frostiness is the effect caused by the uneven surface of the coin, which causes light to reflect in in many directions at once. The uneven surfaces of the coin can be the effect of metal flow, a rough die or a rough planchet. By comparison, mirrored surfaces are very smooth, causing light to reflect directly back to the source.

As for the cameo (frosty) effect found on the devices of some proofs, that's the result of an intentional treatment of the die in which the recessed areas of the die are somehow given a microscopic "roughing up". Perhaps someone else can give more detail about that process.

Longacre

Eureka:

Great explanation. Thanks. Hopefully others will answer your question about the process.

fivecents

The whole die is "roughed up", but only the fields of the proof die are polished to a mirrored finnish.

fivecents

I think mint frost on the business strikes are caused by radial flow lines from the planchet being squeezed by the dies.

Conder101

First we have to define what we are discussing. The term "Mint frost" is sometimes used to mean luster, and sometimes to refer to a cameo contrast. If you are refering to luster then fivecents is correct, it's causd by the radial flowlines from the outward expantion of the planchet. If you mean cameo contrast then MrEureka's eplaination is the one you want.

As for how the cameo contrast is achieved fivecents is right the whole die is roughened and then the fields are polished. The roughening of the die can be achieved in two ways, one is by pickling in an acid bath. The resulting texture of the cameo wil be dependant on the length of the bath, the composition of the die, and the composition and strength of the acid.

I would like to clarify something about cameo contrast from MrEureka's description. He said " mirrored surfaces are very smooth, causing light to reflect directly back to the source. " This is correct if the light is coming in perpendicular to the mirrored surface. But in that case there will be little or no contrast seen betwen the devices and the mirrored fields. Contrast is seen when the light is coming in from the side (lt's say from the right.) at lower angle. Since angle of incident equals angle of reflection the light that hits the fields is reflected off to the left at the same angle and no upwards into the eyes. This same light when it hits the frosted devices it is scattered and part of the light IS reflected up into the eyes. The greater the degree of frost, the more light gets scattered into the eyes. So you get "lit" devices sitting on effectively unlit fields. The classic "black and white" cameo effect.

Eastside

Mint Frost is caused by consumer demand and Right Aid has served it forever even when they were called Thriftys it was an old flavor and probably the no# 3 best selling ice cream behind Chocolate Chip & Mint & Chip............what ...OH.....nevermind


anything that get squashed under tons of pressure is gonna glow with a little haze and I believe the planchets with there varritie degree's of hardness prior to being struck has some influance
to what degree of lustre or lack of that a coin may exibit as well with some of the processes mentioned above.........its a process that is ALMOST impossibe to duplicate or doctor.

Eastside