Devilled Ham is lean cooked ham that is finely minced to form a soft spread for sandwiches, canapés, crackers, celery sticks, etc.
Though classed as spiced, it was spicy in the context of the old days, when some people were almost tempted to classify parsley as a spice -- just the addition of ground black pepper would have made it spicy for them. It was never intended to be eye-poppingly hot or spicy, and is actually quite bland. The usual seasonings are a bit of mustard and cayenne pepper, though some recipes will call for a hot mustard, horseradish, or curry powder, and there's nothing that says you can't pep it up for today's expectations by stirring in some diced jalapeno chiles from a tin.
Some recipes have you start with cooked ham, because most people made it at home from leftover ham. More ambitious recipes have you start with uncooked ham, and then bake it. Most (but not all) recipes have mayonnaise in common; some call for sour cream. Some call for you to start with a commercially-bought tin of Devilled Ham. Most advise you to make the mixture a few hours ahead of time, to allow the flavours to marry.
Some commercial brands are made from pork lean plus pork offal (heart, tongue, etc.)
Underwood Devilled Ham is sold in small tins wrapped in paper. It does not need refrigeration until opened. The ham is cooked in the tins and sterilized during processing.
Underwood Devilled Ham was created by the company established by William Underwood (1787 – 1864.) In 1817, he emigrated to America from England, where he had worked for West and Wyatt (which later became Crosse & Blackwell.) He arrived in New Orleans, and made his way up to Boston, covering large parts of the journey on foot. He set up a business at Russia Wharf in Boston, Massachusetts in 1822. He sold condiments such as mustard, ketchup, pickles, as well as marmalade and fruits, all in glass bottles. Because American consumers didn't trust the quality of processed American food stuffs, and preferred to keep buying British goods instead, he sold his goods to the Caribbean and South America . By 1836, local bottle makers in Boston couldn't keep up with the supply of bottles his business needed, so he switched to packaging his foodstuffs as canned goods by 1839. Once again, he met with domestic resistance, because consumers couldn't visually see the quality of the food goods inside the tins, but the company worked at building trust amongst its consumers.
By the 1860s, the company limited its products to tinned meats and fish. During the American Civil War, his company had a supply contract for the northern army.
When William Underwood died in 1864, his sons took over the running of the business. In 1868, they started developing a spiced ground ham mixture. They called it "Devilled Ham." They developed a little red devil as the trademark, and registered the trademark in the United States in 1870. In 1895 began advertising it nationally. 
Underwood Devilled Ham is now owned by B&G Foods (as of 2004), purchased from its previous owner, Pillsbury, in 1999.
 DiBacco, Thomas V. Made in the U.S.A.: The History of American Business. Beard Books. 2003. Page 104 to 105.
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