I am fairly new to the game and have a couple of questions that I can't find a specific answer to in the core book. Please provide page references if I am missing something.
Page 62 talks about abilities with a zero score and allows you to use some of these but with increased botch dice (those not asterisked). Page 106 states that the apprentice whose arts are 'opened' gains a score of zero in all 15 arts. However I can find no specific rule as to whether such scores of zero in an art are usable. My assumption is that they are not since even those skills which can be used with a zero rating are the more 'natural' skills that every human should have at least some chance of being able to fluke success without having studied or practised them. I would be happier with a definitive answer though.
2, Can any starting mage spend some of their experience on spell mastery skills or do you need to take one of the appropriate virtues?
I am probably missing obvious answers to both questions but they are bugging me so I thought I would ask them here.
Unless specifically stated that you can't, you can spend character generation xp on anything you would spend it on in once the game begins.
Also - many people (with regard to skills) rule that having 1 xp in a skill is enough that you are no longer untrained. The skill value is still 0, but you no longer suffer the extra botch dice and can make use of things like lore skills. You're still bad at them.
Beyond that, consider this another 'yes, like callen said'
Many thanks for the prompt and helpful answers I thought the spell mastery answer would be a yes but I have to admit I am surprised by the zero arts answer. I am not doubting the answer in the least but somehow it feels odd to me that every mage straight out of apprenticeship has the ability to use every art to some degree even those that they have not actively studied. What is the reasoning behind this? Perhaps that the opening of the arts teaches the basics of all 15 arts? Since no-one has mentioned it, I presume that there is no increase in botch dice from using a zero rated art? I would also still appreciate a page reference, if anyone knows it, since I still cant find it and want to be able to show it to others in the book.
Pretty much that, yup. Opening the arts gives you all arts at 0, so it makes it possible for you to use them. Magic theory is what would teach the basics of the arts. Since you have a magic theory score, you pretty much know the basics of all the Arts You suck at casting a Tech+Form combo of 0+0, but you can do it. Take in mind that a startying specialist in a Te+Fo combo is likely to have a total of 8-10 levels in each, for a grand total of 16-20 points to be added to his lab or casting total. Compared to the 0+0 of the untrained dude, it is quite a difference
Ability - 5 experience points to reach level 1, but 1 point is enough to allow default use without extra botch dice. I don't consider 1 experience point enough to choose a specialty, but I don't think there is a clear answer to that.
Art - 1 experience point is enough to reach level 1, so there is no other zero-level except with 0 experience. But you wouldn't have a score of 0 with no exposure to the Art. Just having the Art at all is basically the equivalent of 1 experience point in an Ability.
In response to your second question:
In Ars Magica a score of 0 is very different from having no score at all. A character with an MR of 0 can resist supernatural effects that don't have any Penetration and a character with an Art of 0 can indeed use that art in his/her magic. Put concisely: 0 < 1, no score = can't use.
I hope this has helped clarify the issue somewhat...
You suck at casting a Tech+Form combo of 0+0, but you can do it.