I dip old brass,etc into a ceramic or glass container of muriatic acid. This is sold in gallon jugs at hardware stores as swimming pool cleaner. It's diluted hydrochloric acid. I leave the object immersed for a few seconds then use a glass or soapstone tool to remove it from the acid and place it into a container of water. After that I use a steel wool dish washer pad to further clean the object. Be very careful not to get the muriatic acid on your skin or eyes.
Nice find !! put it in a small container completely cover it with white vineager ,,let soak a day or 2,,rinse, brush with a toothbrush ,,shine it up with some steel wool. Do not do this if you want to keep the green patina.
I dug one of those up under my back deck last week. The dump layer I was digging looked to be 1860's to 1870's based on the bottles (see post Last Night Under the Deck). Note: the pontil'd bottle in my post came from a deeper layer that was presumably late 1850's. The spigot was in a layer about a foot higher up that had slightly newer stuff.
I remembered! This one is a little different, having a "T" shaped handle - with one arm of the T missing. Yours seems to be missing the handle entirely? Otherwise though, almost the same.
It should be noted that even though the layer it was in can be dated to around 1870, the spigot would not have been thrown away until it was worn out or broken. It could have been in use for many years - or even decades!