In a compromise move I just picked up this denarius of Julia Titi which is going to require a bit of work. The coin is not here yet, hence seller photo. The coin is good silver throughout, so the green spots appear to be copper bearing accretions, probably from bronze coins (and coin debris) in close contact for centuries. I have been told that the use of lemon juice (or lemon slices?) will help break down the accretions. But I don't know what to expect in terms of discoloration, nor the steps needed to stabilize the coin after treatment with the lemon. At the moment I plan to remove any residual accretion with microscope and needle, an old method I have used with silver successfully in the past. But first I want to see what can be done with mild acid, if that is the right move. Any tips suggestions or caveats are requested and will be most informative for me. Your thoughts?
It already looks professionally cleaned and appears they stopped for a reason. Probably segments of metal impurities or horn silver that would break or wear down under the surface metal, if removed.
Lemon juice is acidic. Any acidic substance risks turning an otherwise good coin into a porous coin. I really don't get the rush to acid. When it comes to these coins, the harsher remedies should always be the remedies of last resort. Whomever recommends lemon juice as a first resort should be banned for a week. Sounds extreme? Well, so does using acid as a first resort. Ultimately, the safest bet is to do nothing, but if you choose to go ahead and clean, start with the more benign and boring methods first, and move up from there.
I use lemon juice. Sometimes it changes silver to a brighter white, but would rather have that than those crusty spots. You can dilute the lemon juice with some water too. I use round toothpicks to pick it off after soaking for a bit. Real lemon juice is best, not the grocery bottled stuff.
How would lemon juice make a silver coin porous? I don't understand that statement. Silver is a noble metal and has good resistance to corrosion by acids, especially a weak acid like citric acid.
Tough to do on a good silver denarius, but low silver content coins and Alexandrian Tetradrachms with silver can get a little porous if too much lemon juice is on them or for too long. It's happened to me over the years.
Sorry but citric acid won't make elemental silver nor copper porous, and taking in additional consideration that the citric acid in lemon juice is already diluted. The only way it can make a coin "porous" is if the coin was already porous from the start but these porous parts were filled with for example silver sulfide or copper oxide, these do dissolve after a while in citric acid and will make those existing porous parts visible again.
That coin is at 80-90% silver fineness, it will not end up a porous coin if lemon juice is applied - although it will 'brighten' the metal.
My thanks for all your considered opinions. If there was going to be a reaction with citric acid it would not be from the silver but from other alloying metals in the fabric of the coin. If a coin was of a lower fineness, then it is conceivable that a weak acid might make it appear grainy or porous. However, Butcher and Ponting in The Metallurgy of Roman Silver Coinage, report that the coins of Julia Titi under Titus were produced at a putative fineness of 80%. They reported on only two examples of her denarii, one of which assayed at 88.5% silver, and the other at 93%. While it is hard to generalize on such a small sample, the general assay of what is to be expected for her silver should be high enough that a weak acid, such as citric or acetic, is not likely to cause trouble, particularly if it is applied in a spot fashion rather than full immersion. My plan is to apply the acid under a microscope in droplets off the end of a needle or toothpick directly to the spots of corrosion. If that results in an uneven appearance, I may follow with a more uniform wash of the acid, and a thorough distilled water rinse. What is uncertain here is the path to follow if a measure of horn silver should be revealed. Since I do not expect to be working with chlorides I do not really expect this problem. Since the group is much divided on what to expect from such a procedure, I will report my method and observations after the work is done. But thank you for your thoughts so far.
Even to other alloying metals in elemental form like copper (diluted) citric acid won't react in such a short time, only to its oxides or for example metal sulfides. We are also talking about an alloy, the behavior of the metals are different then when they are separated. The metals are homogenous mixed and the layers of metal can not slide from eachother. On the end almost all metals do react but in a very long exposure time, a process called corrosion, I think many people are familiar with it. Carbon dioxide from the atmosphere with moisture will make a weak acid which causes together with oxidization of the metal these beautiful patina's on for example bronze coins. In other words, lemon juice i.e. citric acid or vinegar i.e. acetic acid can be used without problems to clean silver coins for a short period. It will brighten the metals because the oxides and sulfides are getting removed.
I think lemon juice would be fine for this coin, but for something even more innocuous you could try household ammonia. Bob Reis recommends trying it before acids, and I have had some luck with it in similar situations. (But some green deposits require acid.)
A gentle 2 parts distilled water and 1 part ammonia mixture would be my #1 recommendation before any acids too. Please make sure the water is distilled and not tap water . You can buy a gallon of distilled water at the store for $1 and ammonia at Home Depot or another hardware store. Ammonia should be safe on Roman silver up to around 250 CE. Anything newer (ie. billon coins) or bronze coins require a different method. The ammonia is only for 40%+ silver content Imperial coins. Avoid ammonia for provincials as some "provincial silver" may actually be billon.
Ammonia is also not that friendly if compared to acid. But if you use it I would recommend an 10% ammonia solution.
You definitely want to delude it in distilled water, and obviously the coin should be monitored the same as if soaking it in lemon. With either method you definitely don't want to leave the coin overnight. A few gentle quick soaks at first to see if that does the trick, followed by immersion a few minutes at a time, but I definitely would not leave a coin unattended. I prefer ammonia as it's gentler on coins that are less than 80% silver, but still silver. But that's just me.
I've used lemon juice on my Tiberius denarius and Louis the Pious denier with no issues, although it was not strong enough to remove some of the obverse gunk and almost the entire reverse gunk with my Tiberius. So then I discovered the ammonia-distilled water mix and used that with very good results (the LP denier was cleaned just with lemon juice).
Personally, I wouldn’t touch that coin, but for what it’s worth I’ve had good success cleaning silver coins with the baking soda and aluminum foil method.
Household ammonia is a 5-10% solution. Did you mean we should use that straight out of the bottle, or are you advocating further dilution? I've used it straight out of the bottle with no appreciable harm to coins. I don't have a ton of examples of its use though.
