So I just crafted a Skolver Cap. It's the first 5* item I've crafted since the forge system was added.
...But where do I find the 5* fire crystals I need to level it up? I've been through Firestorm Citadel quite
a few times and I've never found one, while I'm practically drowning in lower-tier crystals.
Finding Radiant Fire Crystals.
I wrote a guide about where to find things-
http://forums.spiralknights.com/en/node/108732
The short answer is to find treasure boxes on depths 27 and 28 while playing on elite mode. This means selecting gates that end in arena and clockwork tunnels levels or farming 'Dreams and Nightmares' if you're capable and willing to accept receiving a limited (zero) amount of non radiant fire crystals. Firestorm Citadel is also an option, if you're into that.
It's a frequently asked question. Here's my answer:
http://wiki.spiralknights.com/User:Jdavis/FAQs#How_do_I_find_Radiant_Fir...
My answer hasn't been updated for the newest missions, but it links to data sources that are still valuable.
However, Passunder, there's a lot of weapons which have other differences, like antiguas getting the useful charge, Divine avenger getting two extra holoswords, alchemers getting two extra shots on the charge, it can add up to way more then 15%, the alchemer one being a bit under double damage on the charge, which makes the upgrade more then worth it.
And, on the original question, I've found that the new mission "Dreams and Nightmares" is good, having gotten 30 rads on my last run there, and around that level on the previous one, and the first two levels can go by quickly.
Also, a forge box, if you get lucky, is always a nice boost.
Run the core on elite.
If you have a run speed outfit, use that combined with ele brandish, shadow brandish, a pistol, and then whatever other weapon you want for the first two floors. Switch to a chaos loadout with an ele brandish, shadow brandish, DR, and DBB for the final arena. Very fast, good rewards. The length of the run is highly variable based on how much you have to carry keys and how many arenas you face so don't be discouraged if you have a few lengthy runs, things average out over time. Run seems way faster if you duo it by the way, since one guy can carry a key and the other can clear the way for him.
Also, I don't really agree with passunder's comment fully. 15% damage can be pretty big (the difference between killing a trojan in one charge attack or not), and you also lose out in the way of the charge attack area of effect. Brandishes has smaller and less explosions, bombs have smaller explosion radius, so on and so forth. The armor I can agree with to an extent, but your most utilized gear you should probably upgrade to 10 simply because you use it so often that the small increase you get in survivability adds up over time. That said, I could see the logic in using 4* typed weapons over using a 5* levi for instance. Normal damage is really garbage in this game and it's better to have slightly weaker but correctly typed weapons than focus on building up a few "general purpose" weapons.
As I've said in the past- the solution is to get the radiant crystals before crafting the 5* gear, provided you're relying on that 4* gear. Settling for 4* gear is often great in the short term as it maximizes your strengths when weapons don't get much new but in the long run there's not much reason to just settle at second best.
You get like 10-30ish in FSC. It takes a lot of grinding or CE to purchase the... 470(?) radiants to heat one 5* item to level 10. If you're a person short on time or unable to afford the outrageous cost of radiants, you can just be happy with the 4* version. Last I checked the difference between 4* and 5* weapons is 15% damage, and the 5* armors need only be heated to level 5 for that +1 health bonus if at all.
I can vouch for the 4* path, it's pretty solid.