The issue for Rome was not the reduction in percentage of silver or gold in the coins; the issue was the increased number of coins in circulation. This is what drives inflation. Inflation occurs when the quantity of money in circulation grows but GDP does not increase. As Rome had no way to increase GDP, this meant that (for example) doubling the number of coins in circulation would ultimately halve their value.

Spain provides a case study in why the metal content of coins is totally irrelevant. When Cortez began shipping back silver from the New World, this enabled the Spanish crown to issue lots of new coins - which were 100% silver. Inflation then kicked in because it is the volume of coins in circulation that matters, not what those coins are made from.

Governments can print money but not generate inflation if - and only if - any increase in money supply matches increase in GDP. So an expanding economy can support an expanding money supply. But when governments print more money that GDP growth supports, inflation kicks in.

This is very basic economics 101, and it continues to surprise me that many people seem incapable of grasping such a simple concept but instead imagine that there is some magical "real" value in gold, or silver, or whatever. Anything fungible is merely a token, regardless of how shiny it may happen to be. Remember: we once used sea-shells as money, and calcium carbonate is abundant.