There are two kinds of Spell Matrix the party knows about: Mental and Physical.

Mental Matrices

 A Mental Spell Matrix is a [Talent] --- it costs 4 points/level, just like any other [Talent].  Its rank is often referred to as its "Size."  Each point of size can be allocated to either holding a [Spell Thread] or filtering a single point of [Taint].  If a Matrix is to be used for a spell, it must hold the entire spell --- so you can't combine Matrices, or use a 3 point Matrix to make a 30 point spell cheaper.

 When the spell is cast, the Matrix makes all the [Taint] go away.  This knocks the Matrix crooked: it is now said to be "Skewed" by a number of points equal to the [Taint] absorbed (spell size + thread size).  The [Skew] is applied as a penalty to further uses of that Matrix until it is Unskewed.  See the [Unskewing Methods].

 A Matrix must be attuned to the spell you want it to hold. [Attuning a Spell Matrix] lets you do this.

Physical Matrices

 A [Grimoire] is an example of a Physical Matrix, though a somewhat odd one --- it has adaptations for easy learning and for multiple-spell content.  It would almost be better to say that it's a Matrix Rack, and the text descriptions of the spells are the Matrices.  But more about that once I write real Physical Matrix rules.

 For now, all I'm thinking is that Physical Matrices should unskew much more slowly than Mental Matrices, and should not be reattunable without being ripped apart and rebuilt.  That also rules out the faster unskewing methods.  The unskewing speed and the density of the matrix (points per square/cubic inch) should be somehow dependent on the material used and on the artistic skill of the craftsman.

 There should also be some way to make specialized matrices --- something analogous to Threads for spells.  These must include auto-discharge (for weapons of smiting), obvious spell (for Grimoires), multiple spells (Grimoires again).