if ( mat == MaterialType.InvalidType )
((ItemMultiMaterial) materialItem.item()).createMaterial( mat );
else
{
Field f = materialClass.getField( "material" + mat.name() );
IStackSrc is = ((ItemMultiMaterial) materialItem.item()).createMaterial( mat );
if ( is != null )
f.set( materials, new DamagedItemDefinition( is ) );
else
f.set( materials, new NullItemDefinition() );
}