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To answer the question " how effects magnetism on goo" its important to know what goo realy is , further i think this effect would be in a biological or chemical way, i dont think there are physical effects.

My question would be "how is it possible to have magnetism on Atys?" but that would be off topic.

Last reference to black magic in game (aniro, i don't know other) : marung horongi, zorai constitution (before merge), and wize quest in zora (Companie of Eternal Tree accuse Black Circle to use it ; you can verify that easily). I found old reference by players too (few guild revendicated be Black Wizard, and other guild sugger they fight them in distant past) but i think it's before beta. But Black Magic has not got direct link with goo. Indirect link : "maybe" it can create or destroy goo but like it's very forbidden, no experiment possible... (and not implemented ^^).
In aniro, during an event, Thesos was contaminated by goo. For fight it, toon plant vegetables who "aspirate" goo in ground and water and want burn it after, in forges of the Fyros. But event team say "no, forges haven't got natural fire, so goo is not destroy". So they throwed vegetable in goo in zorai land.

And repeatedly animator says (in aniro before the merge) : "there are not natural fire on Atys, except very uncommon phenomenon, like Coriolis". I confess, i don't like this. That our fireballs are "magic fire", ok. In Forges of the Fyros and in pyr's gate, why not. Special fire for Flamming Forest, probably, because if you try real fire and sawdust (as the sand of the desert is supposed), you obtain big conflagration.

But, if you enough warm a material like wood or sawdust , you obtain fire, real fire. And homin can make that. before merge we have campfire, they don't seem magic. And for event cited (fire destroy last spot of goo), this fire is created by carelessness of homin ! like campfire, maybe ! So, i think goo can be destroyed by very very hot fire. Difficult for homin to create this, because they don't have utility and so, they can't have technic.

The Forge of Pyr is for armor and weapon with bark, amber, sap, etc : material that do not need a big heat. Not like iron or steel (unknow on Atys outside Karavan). I think it's for this reason that tryker's box destroy goo : it's warm goo very hot, like in solar furnace ?

I have to precise that, in my opinion at least, this "Tryker Box" experiment has doubtful resulsts (in a roleplaying point of vue).
OOC it seems to be a player event, even if managed by an event team, being long ago (maybe previous winchgate) i'm not sure it can be treated as "real" in our virtual world of Atys :)

Another addition:

On the old temple of Ma-Duk outside of Zora it said:

"The Kamis and the Zoraï

In the old days, the Kamis had taken refuge in the jungle after the great fires, where they met the Zoraïs and entrusted them with the sacred mission to fight the Goo. The spirits of Nature developed very close ties with the Zoraïs, leading to the early devotions to the Kamis, who were the living incarnation of the pure energy that created Atys. The Kamis created the masks linking them to the Zoraïs, as a symbol of this spiritual bond. The Kamis initiated the Zoraïs into the secrets of magic. The first magicians appeared on Atys, performing miracles.

Then the Zoraïs' high Magi began to study the arcane mysteries, using sacred writing to transmit their knowledge. They learned the secret powers of lightning, and created the first peoples' magic, the magic of electricity. Using their magic and their knowledge of magnetism, the Zoraïs fought the Goo that was eating away at the forests of Atys. In the name of the Kamis, they bravely battled this plague. They were wary of the Karavan, who led homins away from the path of Enlightenment.
Their unshakeable faith has made the Zoraïs the sacred warriors of Ma-Duk."
http://en.ryzomnomnom.com/wiki/Kami#The_Kamis_and_the_Zora.C3.AF

The same had been written there in German:

"Mit ihrer Magie und ihrem Wissen über den Magnetismus bekämpften die Zoraï das Goo, das den Dschungel von Atys befiel."