Milk Spots

[pendragon1998]

I've been thinking for a while about milk spots. I hear people saying that they are a result of a bad rinse at the mint, but to my eye at least, they seem like residue that has splattered onto the coin (i.e., they consist of small circular areas of varying size, much like the pattern of moisture on a pane of glass one has sneezed on). I would expect residue from a rinse to either film up a large portion of the coin's surface or be concentrated along an edge, where the liquid ran before being dried off. I've come up with an alternative hypothesis.

 

I recently watched a little show about the mint on tv. During the show, a worker was shown blowing off dust from a coin prior to encapsulation using a compressed air nozzle. The air nozzle was attached to the wall by an air tube, presumably leading to a large air compressor on the building exterior.

 

This made me wonder if what may be happening to cause milk spots is that the compressed air is not being properly dehumidified, leaving small amounts of water in the air stream. We had a similar problem here at a lab I worked in at the university. We used a compressed air tap to blow-dry small equipment. When it was very humid, the air would occasionally spray small droplets of water out with the air. If this hit the lab bench, it would leave tiny beads of water on the surface.

 

This would match the pattern of milk spotting I see on silver eagles. What are y'all's opinions on this hypothesis of mine? Could this be what's going on?

[Oldtrader3]

You could be right. Where I worked, our labs and clean rooms used several levels of treatment for compressed air, consisting of a coalescing filter, a dryer, a 50 micron filter and a 0.2 micron final filter to assure that air was bioburden, moisture and particulate free. Most labs probably use a similar setup but I doubt that the mint uses anywhere near this level of filtration and drying for compressed air.

[Stanley]

That has been my theory all along.

 

Compressed air unless it is dehumidified and filtered correctly does contain moisture which is drawn from the atmosphere. It can also contain droplets of oil residue from the compressor or any build up in the air delivery system ( air pipes and hoses.) I am very familiar with this from spray painting automobiles. The moisture can cause costly problems for spray painters and can foul and clog air tools shortening the usable life of the tools. Anyone who has ever drained an air compressor tank of excess water would know the water is always white like watered down milk.

 

I would think the grading services are aware of this.

 

BTW If anyone needs any further information on air compressor, dryers, and filters I can provide more information.

[bsshog40]

That sounds like a very reasonable conclusion. Any type of unit storing compressed air will accumulate moisture. This could be a very good cause of it. You would think that there would be more intelligence used during any type of this cleaning process when compressed air is having to be used!

[physics-fan3.14]

What is the chemical makeup of these types of spots? Milk spots are non-reparable, they cannot be removed. This is why many people have theorized that they were a result of a process before striking (the traditional theory of milk-spots). Also, when did this type of compressor/blowing process start being used? Milk spots started being a problem in the 40s or 50s (I think). Someone with more knowledge or research could probably compare when rinse processes were used or when the blower came online to when the spots started appearing, and make a strong argument one way or the other.

[Dwaine]

I have had some proof sets from the 60's with milk spots on them.

[ldhair]

It's my understanding that it's in the surface of the coin, not on it. If it were on the surface it should dip off.

[Larry Dreher]

I've seen these on Chinese silver pandas. I doubt they use an air compressor in their process but I could be wrong.

[physics-fan3.14]

I think some of the confusion is in the use of the term. As I have understood it, milk spots only refers to one specific kind of spotting, the kind commonly thought to be due to a pre-strike washing of the planchets that is not removable. I thought the milk spots were due to a specific kind of wash (Ivory soap, I think) and when the mint realized the problem, they changed the wash. So, milk spotting, as I understood the term, does not occur on todays coins. Other spotting, of course, does still occur. Other kinds of spots can be due to air compressors, or a myriad of other causes.

[ldhair]

I think some of the confusion is in the use of the term. As I have understood it, milk spots only refers to one specific kind of spotting, the kind commonly thought to be due to a pre-strike washing of the planchets that is not removable. I thought the milk spots were due to a specific kind of wash (Ivory soap, I think) and when the mint realized the problem, they changed the wash. So, milk spotting, as I understood the term, does not occur on todays coins. Other spotting, of course, does still occur. Other kinds of spots can be due to air compressors, or a myriad of other causes.

I see it much the same.

[John Martinez]

I agree with your therory; however, I wonder if it is limited to the Mint? I have several rolls of SAE's from several years. Not a single milk spot. I purchased a holdered set of SAE's one from each year. EVERY SINGLE ONE has milk spots to some degree. BTW - I'm talkin' MS, not proofs.

[just-john]

i thought it was caused from the white liquid used to cool the dies that comes through even after they are cleaned/rinsed after minting ? I have a HASS CNC machine and the coolent

used on the end mills/cutters looks like milk even when you wash real good a part like stainless steel

or titanium sometimes the spots from the lube come back.