ToME WikiHmmm. Is this page a spoiler?
The XP needed to gain levels for your item: (0.25 times x time x) + x + f
where x is the cost of all flags with a variable value (such as speed), f is the cost of NEW flags with no variable value.
When you re-improve an artifact the old flags, counting at the old pvals, count for things you already had for free; you pay only for the new stuff.
It doesn't matter to XP in which order you add flags/pval, but adding as much as possible at once is popular because of the high magic essences per change cost...
Because of the formula, 1 artifact of +60 speed would cost much more than 2 artifacts of +30 speed; and in turn they'd cost less than 5 artifacts of +12 speed... since it relates to pval regardless of the number of flags, it is quicker to have 2 rings that have both the luck and speed flag at +10 than to have one ring at +20 speed and the other at +20 luck.
Since the pval affects all things, and you might want to say have no more than +12 STR on an artifact because it maxes your STR, it is convenient to have one artifact for maxing all 6 stats and add flags until all good ones are done but never raise the pval beyond what you need to max your stats. Having only one such artifact is ideal since it lets all your other artifact equipment slots evolve to infinity.
Infinity? Yes. Because pval is limited only by your patience... at least in 2.2.7
You can save some small amount of XP by applying a curse to the item (and remove it). This is covered in the First Artifact spoiler, but it's usually not that useful.
Samwise asks:
Do you have to deliberately allocate artifact XP in order to bump up pval, or is this just supposed to happen automatically (when you upgrade flags)? Does the XP used to bump up pval then disappear?
So far, I've been working on an artifact that I made from a Cloak of Air -- even after earning 100K in XP on the artifact, I've yet to see a bump in pval, and the artifact has only a few sustains, and a +1 bonus to strength currently added to it. Could the Cloak of Air be screwing this up in some way?
@Samwise: You have to pay for pval increase on the artifact creation screen.