JoB that’s exactly it! They may have a lot of sheep, but not so many that she would have to spend her life drawing the sheep husbandry staves, if they were permanent. In that case they would also be painted into the post before building the fence, when the mage could work at a workbench rather than kneeling on uneven rocky ground.
Also, the reason they're not permanent cannot simply be bad choice of material, because then we'd see them getting
carved into the posts before the fence is erected. So, whatever the details are, it's something with the
magic involved that limits their lifetime.
... and that's as far as we'll get the matter investigated, since Minna did not ever write down, much less publish, a Complete Guide to MagicSSSS, I'm afraid. Of course, one can theorize that Icelanders might be worried that
too much human-made magic could draw the scorn of Iceland's
other magic folk ...
At the very least the Icelandic ships [...] would be covered in them.
Hm. Who says they're
not - just on the
inside of the hull, where they (and those repainting them when necessary) enjoy a longer life?
(The outsides are left to the
original staves of the ancients.
)
And I find it hard to believe that even the Swedes and Danes would remain skeptical if there were items that clearly have lasting effect.
I guess that raises the question whether the seiður hand magic-warded items
to others, or make them only for their own folks' use (which would be greatly "helped" by a short lifespan of the effect). It's simple to disbelieve in stuff you never get to see working
in person ...
(Reynir apparently is willing to
share, but
look what the result of the uninitiated handling the stuff is.)