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<p>[QUOTE=&quot;Roerbakmix, post: 7785382, member: 100731&quot;]Some of you probably know that I like to restore ancient silver coins. Over the past few years, I&#039;ve treated hundreds, both my own coins, but for other collectors as well. I prefer chemical cleaning above mechanical cleaning (which may result in scratches), though usually, some mechanical cleaning is required. </p><p><br /></p><p>My usual process is as following:</p><ol> <li>Identify the material: coins with high silver content are easier than billion coins, as silver is relatively inert (i.e. not harmed) by most chemicals, while the copper of billion coins is easily dissolved. </li> <li>Take a macro photograph of both obverse and reverse. </li> <li>Identify the deposits: there are many types of deposits on coins, that can be sub-categorized as well. It is common to encounter more than one type. However, the most occurring depositions are:<ul> <li>Silver chlorides, or horn silver. A black to grey deposit</li> <li>Silver sulphides. Black to grey deposits, sometimes difficult to tell apart from silver chlorides. </li> <li>Copper oxides. Green deposit</li> <li>Earthen encrustations</li> <li>Iron oxides. Brown deposits&nbsp; </li> </ul></li> <li>Define the optimal cleaning strategy </li> <li>Predict how the coin will turn out (i.e. avoid pitting, unwanted colour differences, or other side effects). </li> </ol><p>Cleaning is relatively easy if you know what you&#039;re doing. For example, silver chlorides dissolve easily in sodium thiousulphate (anhydrate), without harming the underlying patina. Copper oxides dissolves easy in weak acids, such as synthetic citric acid. </p><p><br /></p><p>Iron oxides are notoriously difficult however. I usually boiled the coin in concentrated solutions of synthetic citric acid, brushed it, poked it with a bamboo stick, and repeated this procedure until the deposits were gone. Depending on the amount of iron oxides, this procedure could take hours of cleaning. Also, even minor deposits required me to immerse the entire coin in the solution, resulting exposure of the entire surface to potentially harmful chemicals. </p><p><br /></p><p>So I needed a stronger chemical that did not harm the patina, nor the silver. After reading around, I tried concentrated (30%) hydrochloric acid <a href="https://www.cointalk.com/threads/proof-of-principle-removing-iron-oxides-from-silver-coins.379500/" class="internalLink ProxyLink" data-proxy-href="https://www.cointalk.com/threads/proof-of-principle-removing-iron-oxides-from-silver-coins.379500/">with dramatic (positive) results</a>. And today, a friend of mine who works at the lab gifted me two mechanical pipettes (2-20 uL and 20-200 uL). Of course, I had to try it out, and looking for a silver coin with only minimal deposits, I remembered my Azes tetradrachm:</p><p>[ATTACH=full]1336017[/ATTACH]</p><p>Note the iron oxides between the forelegs, and underneath the horse. I then aspirated a tiny amount of 10 ul (1/100th of a mL!), and placed this small droplet at the deposit:</p><p>[ATTACH=full]1336021[/ATTACH]</p><p>Much to my frustration, the droplet quickly (i.e. this timelapse took about 30 seconds) &#039;flattened out&#039; and did exactly what it not was supposed to do: expose the nice areas to the (quite aggressive) HCl. The iron oxides remained unchanged. </p><p><br /></p><p>However, after rinsing the coin, and brushing it with a soft towel, the iron oxides were easily brushed away (that is, most of them). </p><p>[ATTACH=full]1336025[/ATTACH]</p><p><br /></p><p>So, some lessons learned:</p><ul> <li>Smaller droplets are required. Next time, I&#039;ll try 2uL and see how it goes. </li> <li>Perhaps protecting the nicer areas, e.g. using vaseline</li> </ul><p><br /></p><p>Curious to hear your replies.[/QUOTE]</p><p><br /></p>

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