Hi jim1:
Thank you very much for all of the information! While I had heard at one time that tintypes were in reverse, I had forgotten that fact and was surprised that both parents did not have wedding rings. Thank you for mentioning tintypes.
The curator thought that the tintype was from the late 1860's, possibly as late as the early 1870's. My interest in dating it is to determine who the children were. If it is from c1872, then we know the children's names. If it is from c1875, there should be three young girls in the photo. (I am not doubting the curator's estimation, as I highly respect his expertise. I posted the photo out of curiosity.)
Have to say it does looks typical 1870's although that dress material isn't but it could be an ethnic pattern.
Patrick and his wife were born in Ireland and lived in Dundee, Scotland in 1869 (when they were married). We believe they sailed to New York in 1869 and by 1870 they were living in a city in New York state with whom we believe to be two female relatives (possibly aunts of Patrick's wife).
We have a few more (later) photographs of Patrick and his beard is shorter, but not gone.
Thank you again for sharing the information; the RootsChatters' knowledge still amazes me.
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I didn't think of asking: would anyone be able to guess the childrens' ages, please, in the photo? (The three daughters were born in March 1870, September 1871 and September 1874.)
Thank you.