Common type of ancient coin, probably Constantine or one of his kids. Perhaps @chrisild or another monitor could move it to the Ancients Section and they will talk your ear off.
This one looks pretty dirty, so your first step should probably be to soak it in water a couple of days, taking it out periodically and using a toothbrush on it and changing the water. See what that gets you.
I am sure the ancient folks will tell you that. Sure they will say, start with a long soak in distilled water. But I would wait for there input on your type of coin.
The reverse of your coin is GLORIA EXERCITVS; Two soldiers helmeted, stg. facing one another, reversed spear in outer hands, inner hands on shields resting on the ground; between them one standard. The mintmark is SMTS[?] and it is from Thessalonica. It is a Constantinian issue and you would need to post the obverse for a complete ID.
Actually, a lot of that dirt looks pretty loose. Even before putting it in distilled water, you should try taking an old toothbrush to it. Try cutting off the nylon bristles close to the base of the brush to get a little more "oomph" out of them.
The single-standard GLORIA EX centenionales were introduced in 335 and were continued through 340 (and beyond, some places - additionally, this reverse was officially "muled" with both the city commemorative obverses - "VRBS ROMA" and "CONSTANTINOPOLIS") so you have 5 separate people with 8 offices between them, plus two allegorical personifications, any of whom who might be on the obverse - 3 of them could be either Caesar ("crown prince") or Augustus (emperor) depending on whether this piece dates to before or after the death of Constantine and the elevation of his 3 sons in 337. Without seeing the obverse, the information that has been shared here so far will be all you'll be able to get, aside from a list of possible people: Constantine I, Augustus Constantine II, as Caesar, then as Augustus Constantius II, as Caesar, then as Augustus Constans, as Caesar, then as Augustus Delmatius, Caesar VRBS ROMA CONSTANTINOPOLIS
http://www.forumancientcoins.com/dougsmith/dd02.html You might find some value in the above link and the page reached by clicking on 'Return to Types Index' at the bottom. I made the page for the benefit of people with common uncleaned coins and ranked your type at #2. There are, as mentioned above, several rulers and several mints so you can tell nothing without the obverse as well.
Distilled vs. tap water... for most uses, tap water works, and if you are excessively anal (short for analytical?), you can do a final rinse with distilled water.
Victor - the CT crew, including the person posting, may not be as familiar with the minutiae involved with these LRB's as are you and I. I will admit to perhaps presuming a lower level of knowledge than may be the case, but when I write for an audience which includes those who probably know little to nothing about ancient coins, I will occasionally oversimplify. Just like the term "Limes Denarius" makes no sense, but is an accepted term in the marketplace and one that some of us have been forced reluctantly to adopt in the name of simplicity of description, "Official Mule" also really doesn't make any sense. However, since the person who encounters for the first time a Gloria Ex or a Vota reverse on a city commemorative will write-in saying "what the heck is this thing? do I have a mule?", the term "official mule" is generally explanation enough to suffice for the moment. The full explanation for the existence of these pieces entwined, as it is, inextricably with the complex politics of the rivalries between the successors to Constantine can come later.
True, but for prolonged soaks, most serious uncleaned coin cleaners, a.k.a., bottom feeders, use distilled water. The are a couple of reasons for this, but the main reason is that most US tap water has been chlorinated. Most bottom feeders live in mortal dread of bronze disease; dread that is--like a typical New Yorker's fear of bedbugs--possibly out of proportion to its actual likelihood, but nonetheless exasperated by the thought of exposing an ancient coin to even a whiff of chlorine. Nasty stuff, that chlorine. Since a gallon of distilled water usually costs less that $2 and can last for months, I don't think it's a question of being anal. I think it's more like a neurosis or mild paranoia.
I agree with neurosis or paranoia. Having an avatar of an ancient coin, I'm sure that NONE of your coins have EVER been cleaned.