Bubbly Fields on Merovingian Coins: The 'Peckham Hoard'

Over on social media some coineys were getting excited a while ago about an old report on the small hoard of seventh-century English and Merovingian gold coins from East Peckham, found and hoiked out of an archaeological context by metal detectorists ( Chris Hunter, 'Ancient gold from secret site near East Peckham could be worth more than £10,000 [...] Ancient find declared 'treasure' at inquest' Kent Online 14 July 2022) Oh, how exciting, eh? But their interest was not focused on the "numismatishness" of the find as much as the treasure-hunting aspects. An archaeologist would look at the context and content of the assemblage. I thought the coins figured in the "hooray, well-done-bet-that's-werf-a-lotta-munny" newspaper article looked odd and queried it. Looking at one of the coins shown in the photos I asked 

Why does the field of the design look so rough? Rusty dies? Is this normal in gold coins of the period? Presumably gold content high, so it'd not be a post-deposition differential leaching.

This was answered

Rory Naismith @Rory_Naismith 22 g.
A degree of roughness is normal, and arises from a combination of how dies were made, and the likelihood that they were not used constantly, so could build up rust.

Having seen not a few of these coins myself in a former life, I am not entirely clear what is meant by this being "normal" and why not using a die for (well, how long?) would allegedly lead to rust-pitting. Also "how the dies were made" seems not to be a good explanation to me. The sinking of the lettering and designs into the face of the die would presumably have to be followed by a filing of the face to remove unevenness and burrs at the edge of the depressions, as we can see in another coin from the East Peckham hoard shown in the same article.

Paul Barford @PortantIssues
Hardly "normal" for the coins of this type actually on the market (and we now have lots thanks to detectorists ripping them out of the archaeological record), most of which have perfectly smooth backgrounds. A few obverses have "bubbly" roughness - why? For example here (Coin Archives)  

As the backup to what I was saying about smoothness/roughness see here (coin archives), here (ac search) and here (Sutton Hoo purse). Smoothness would be important to the user of the coin, as in a bullion economy visually judging the colour of the metal would be important for determining purity (Western European gold at this period was initially very pure, becoming more debased throughout the seventh century).

I do not know whether these technical aspects and the periods of use and disuse of the dies are covered in Rory Naismith's forthcoming book 'Making Money in the Early Middle Ages' ('An examination of coined money and its significance to rulers, aristocrats and peasants in early medieval Europe').

It is worth noting that this "Peckham Hoard" (Tonbridge and Malling) was found and recorded as loose individual items, and much of the PAS description is devoted, instead of the archaeology, to whether they "constitute potential Treasure under the stipulations of The Treasure Act 1996". The objects are four gold coins, KENT-687A04, KENT-672C14, KENT-83554C and KENT-83515A. and two lead weights KENT-689A2D and the similar KENT-688E85 apparently found in association with them.