I'm looking at an effect opposite of the spell Hauberk of Sublime Lightness from HoH:S.
I've looked at the guidelines and searched the forum, but the closest I get is a thread about MuTe base4 to change the volume of stuff.
I consider making something weigh more than normal to be highly unnatural so I don't think it's far fetched to use MuTe base 4 for increasing the weight of something by 8 times even though the volume does not increase. A 25 lbs chain hauberk that suddenly weighs 200 lbs would be kind of troublesome and a sword weighing over 20 lbs would be hard to swing.
And then what about an additional magnitude for exstra weight?
Not an x8 increase but an x64 increase.
A full chain armor weighing close to a ton would crush most grogs.
Granted there are easier ways to disarm an opponent than using a base 4, +2 magnitudes for voice, +1 for duration and +2 for affecting metal, but the basic idea is sound.
the principle is sound but the result is that increasing the density of an object so it weighs eight times as much ends up being the same level as increasing the size of an object so that it weighs eight times as much.
To me the density increase seems more unnatural than the size increase. But not much more unnatural, I think that I'd rule in your favor if you wanted to do this in my game. It isn't an unambiguous mater.
I also think the basic idea is sound.
Increasing density for me is definitely and significantly more unnatural than increasing size; a big suit of armor is just big, unnatural for this suit of armor but otherwise not extraordinary. But iron twice as dense as lead does not exist in nature at all.
Do NOT set that precedence. Stick to doubling the effect for each extra magnitude, NOT increasing it by square, or you will end up breaking the spell you can make with the guideline.
Otherwise, lets say add 4 magnitudes -> 88888=32768.
Cast it on someones helm and, well, squish, when a 1kg helmet suddenly weighs 32 tons.
Basically a level 45 spell that autokills anything it´s used on. Make variations on it for various materials and you have a oneshot autokill spell for anything(because a person or creature getting that kind of weight increase with zero change in structural integrity is not going to end up healthy(or alive)).
Stick with doubling -> 82222=128.
Still plenty useful, but not autosquishyness at least.
It may need an extra magnitude(does the extra volume version of the spell maintain the same weight or not is that main question, if the volume increase increases weight according to size, then the spell needs a magnitude extra for just doing one of the two, otherwise it might be fine), but it is probably close enough.