1999 Proof Set Toning Quesion About Value

Hi drewmc1 Welcome to the forum .

Your coin dealer is right , in most cases unless the toning is very ugly it won't affect the value of the set .

There are collectors who seek out toned coins , but in most cases a mono tone light golden brown color will not be that highly sought after thus the rise of value on those type of coins would be negligible .

The type of toning which carries a nice premium to those who collect them is very uniquely colored coins each series has there own distinguishing colors which are coveted by the toned coin crowd.

contrary to popular belief mint packaging is not element proof , in fact in most cases it is far from it .

the golden color you mentioned is more than likely a combination of humidity and sulphur gasses .

how was the set stored , in a safe with documents and other packaging ,card board ? airtite when the door is closed ? any silica packs ?

all of these things can help coins tone , an airtite safe with card board ,paper and other sulphur based papers concentrates the gasses, humidity also helps those gasses penetrate the mint packaging and the less than adequate plastic cases that the proof sets are contained in . many types of woods also emit gasses that will tone coins .

and then its also possible that the wash that the coins received at the mint may have been contaminated and the brown or gold color is a result of that .

There are just way to many variables to point to one cause or the other ,there are many things that impact how a coin survives storage for many years .

If it helps any my 1999 proof set also had some golden tone to a few of the coins , but it will go no further , as soon as I saw it they went from mint packaging to airtites the process has stopped .