Counterfeiting in the Middle Ages

"Forgery of our king's coins or his seal is an unforgivable offence." So states the common national law code (Landsloven) introduced by Magnus Lagabøte (Magnus the Lawmender) in 1276. Although we have no confirmed evidence of counterfeiting of coins from this period, we assume that there must have been some instances. The explicit prohibition in itself proves that the phenomenon was not unknown. 

In 1910, some road construction workers found a coin hoard in Kalfarlien in Bergen. The hoard contained 1800 coins, all of the same type the familiar penny of Eirik Magnusson with the Norwegian national coat of arms on the obverse. The coins from Kalfarlien contain practically no silver and, in general, are struck rather carelessly. These coins must be counterfeit pennies from the relevant period. 
A find at Alstad in Toten, from around 1820, contained 1600 counterfeit coins. These too were pennies of Eirik Magnusson's type but with the royal crown on the obverse. At the time, they were referred to as "black money". Forgeries of Duke Håkon's pennies (1280-1299) were found alongside the counterfeit black pennies. The Toten counterfeit coins are particularly poorly made and one half the weight of the genuine coins. Unfortunately, the man who found the coins had most of them melted down. Rumour said that the amount of silver he extracted from these coins was enough only for fitting on a tobacco pipe. 

Some popular accounts describe kings as counterfeiters because they mixed copper with the silver. In the "Divine Comedy" Dante lets the French king Philip the Fair be tortured in hell as a punishment for having made adulterated coins.