For smelting ore, each +1 of the Gloves is a 2% increase to the chance of success.
For smelting metal items, each +1 of skill appears to give an additional 0.66% to the ingot return.
As the return for Grandmaster Mining is 66%, it requires either the +3 or +5 gloves to reach 66.67%, and get back the most ingots from items made with multiples of 3 ingots. Similarly, the way the numbers factor, one gets one more ingot with the +5 gloves when smelting 25-ingot items (plate tunics, chaos shields, order shields), than with +3 or lower gloves (69.3% vs.67.97%, with 68% needed).
This is best seen in the use of the +5 gloves, where most items return 69.3% (or whatever whole ingot amount is closest yet less than this precentage). At Grandmaster real skill, There is a 1 ingot difference between +3 & +5 gloves when dealing with items requiring 25 or 28 ingots, and a +1 ingot difference between +1 (or no gloves) & +3 (or +5) gloves, when smelting items made with an ingot amount that is a multiple of 3, as even 101 mining falls short of the 66.67% target for 2/3 ingot return.
