Urza, Lord Protector and The Mightstone and Weakstone melded into Urza, Planeswalker.
Steps
Activate Urza, Planeswalker's third loyalty ability by putting no loyalty counters on it, creating two 1/1 Soldier artifact creature tokens.
Activate Krark-Clan Ironworks twice by sacrificing the two Soldiers, adding ({C} magic symbol) ({C} magic symbol) ({C} magic symbol) ({C} magic symbol) .
The Soldiers die, triggering Dross Scorpion twice, untapping The Chain Veil and any other artifact you control.
Activate The Chain Veil by paying ({4} magic symbol) and tapping it, allowing you an additional activation of your planeswalkers' loyalty abilities this turn.
Repeat.
Results
Infinite activations of loyalty abilities of all other Planeswalkers you control.