In case you haven't heard...

[Lilah]

I have a lot of family in the USPS and am patiently waiting for these to "debut"!!

http://earthsky.org/astronomy-essential ... 3131c612c3

[Deane Clark]

Cool! I didn't know "eclipse stamps" were a thing. I may have to pick up a sheet for next year's stamp supply.

[Lilah]

I know, these look awesome. My sister said they are really cool! I will likely buy extra so I can use some and save some.

[TrenaJ]

Very very cool. I will have to get some as well...never to be used.

[markjob]

I have a few sheets on order at the local PO. I have friend on staff.

Mark

[bkerr]

Eclipse stamps go on sale tomorrow 6/20.

[markjob]

Got mine!

They are very cool.

[Deane Clark]

I know.... “Why is he resurrecting that thread from 2017?”

Anyway, I have recently received a couple pieces of mail with the eclipse stamp on them, and I’ve found that warming the stamp with your finger or by breathing on it no longer makes the moon show up. Has anyone else noticed this? I’m kind of glad I never bothered to buy a bunch of them now that I know this.

[Lilah]

Just checked mine and they don't work. I just have one sheet that I received as a gift.

[Deane Clark]

Well, at least they still work as stamps!

[Dale Smith]

There is a Wikipedia article on this stamp.

https://en.wikipedia.org/wiki/2017_Tota ... ipse_stamp

It mentions that the USPS has said UV (ultraviolet) light can cause the thermochromic ink to degrade so the stamps should be shielded from sunlight.

I find this topic interesting because I am an ink chemist. While I have never personally formulated inks containing thermochromic dyes, I do not find this sensitivity to sunlight surprising. One of the common classes of thermochromic materials are leuco dyes. These are dyes that switch between colored and colorless forms. Many of the common ones are based on triphenyl methane dyes, which are cheap and very widely used in the printing industry. They have notoriously poor fade resistance to light and heat.

[Lilah]

Hmm, well mine were in the original envelope and in a dark drawer. So, they were definitely shielded from sunlight!

[Dale Smith]

Note that I said light and heat. Heat could encompass both rapid changes due to elevated temperature or slow changes at room temperature, such as in a dark drawer.

Another possibility are pH changes. Many triphenyl methane dyes can be used as pH indicators (materials that change color with changes in acidity). As it ages paper tends to create acidic decomposition products.

However, I must add that without knowing the specific thermochromic dye used all I can do is speculate.