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Here a small guide how to use upgrade cores, wich u will get and what are the chances and penalties.
Upgrade Cores
Low Core: Increasing stats on equip lower then Titanium. Un-equip the item you are upgrading, right click on the Low Core and then left click on the item, if it's successful then your item will be +1. (Assuming it was 0)
Medium Core: Increasing stats on Titanium or lower graded equipment and are used in the same manner as a Low Core.
Upgrade Core High: Increasing stats on the high grade equipment like Osmium, Shadow titanium etc... Just like Low and Medium Cores, right click to use.
Upgrade Core Highest: The rarest of the cores, only core that can be used to upgrade Mithril grade equipment.
Enhancement Failure Penalties
+0 to +1 - Fail = nothing happens
+1 to +2 - Fail = nothing happens
+2 to +3 - Fail = nothing happens
+3 to +4 - Fail = nothing happens
+4 to +5 - Fail = -1 to +3
+5 to +6 - Fail = -1 to +4
+6 to +7 - Fail = -6 to +0
So, be very very sure of your decision when you upgrade to that +7 rank, because it fails more often than not.Items can NEVER break
Hope this helps when deciding to upgrade your stuff! ^^