A couple things I'm working on that will be added to the core shortly.  Note that "substance" now refers to the subject of mend, shape, conjure, shape, and animate spells rather than "material" which specifically refers to the materia required for casting.

Strength, Mass, and Size
   The Size rating of an object is the Strength score needed to lift it.  Most of the time this is sufficient detail for the task at hand.  Nobody wants to play Home Work The Roleplaying Game.  While quantifying and tracking the weight of everything is undesirable it is often useful to know exactly how much a character can lift, carry, and how much they weigh.  At the most basic, a character can lift a weight in pounds equal to their Strength squared.  Thus Size is the square root of an object's mass.  It get's a little trickier when it comes to density and volume.  Assuming roughly the density of water two-thousand pounds will fill a cubic yard (or pace).  Metal and stone are about ten times as dense (assuming it's not just a solid block anyhow) and gases one tenth that density.   At any rate a two hundred pound man's volume is about a tenth of a cubic yard though he'll need about two cubic yards to move about and fight.  Of course, Strength isn't the only indicator of a person's mass, Agility is largely about Strength to mass advantage.  Due to the square / cube law, whereby surface area increases at the square and volume at the cube, this relationship is not entirely consistent as size increases.

Lift = Strength2

Mass = n x Strength / Agility
Where n =
Tiny      27 - 64
Small      64 - 125
Medium   125 - 216
Large      216 - 343
Huge      343 - 512
Gargantuan   512 - 729
Colossal   729 - 1000

Size = √weight in pounds


Substances
   The magical methods effecting various substances require some definition of the substances which can be shaped, mended, conjured and animated.  Density is used in the loosest of terms here to provide a mathematically simple relationship between mass and volume.  While it might be possible to look up actual densities it is not particularly desirable as doing so delays play and is probably motivated by a desire to win some argument or another.

Aether
   The stuff of magic has no tangible presence in the mortal realm.  For the sake of practical limitations it has a density one tenth that of water as it is much like air or mist in its own plane.  Spirits and spirit objects have a density one fifth that of water.  Aether is shaped to create barriers in that plane.

Air
   Air has a density of one tenth that of flesh or water.  This means that it is easier to affect a larger volume than would be with most other substances.  The primary trait of air is movement.  Air can push and lift and deflect.

Clay
   A specific variety of dirt that is maleable and elastic when wet that can be dried and fired to create pottery containers and sculptures.  Clay is five times denser than water.  Clay objects are very likely to hold their form.  Wet clay has an armour rating of one tenth its size, dried clay has an armour rating of one tenth its size.  Fired clay has an armour rating of one fifth its size.

Darkness
   Magical darkness is a substance unto itself, on tenth the density of water, like the air.  Darkness obscures and cools.   Darkness is conjured and shaped to create illusions.

Earth
   Or properly speaking, dirt has a density five times that of water.  Dirt is maleable but more structurally cohesive than sand due to the high content of organic material and water.

Fire
   Fire burns, its very nature is to consume and destroy.  Fire is often shaped or conjured to attack.  It causes damage equal to its Size to everything it comes in contact with but it also grows as it destroys.  Mending fire extinguishes it.  

Flesh
   Organic animal mater, living or dead has a nominal density equal to that of water.  Flesh is mended to heal wounds, conjured to create food, and shaped to cause harm or alter appearances.

Gas
   Like air and fire, Gas has a nominal density of one tenth that of water.  There are flammable and toxic gases.

Light
   Though it is weightless, light has a practical density of one twentieth that of water.  Light illuminates and can burn like fire.  It dispels illusions.

Mist
   A diffuse mixture of air and water is useful for concealment and suppressing fires.  Mist moves faster than air but slower than water.
 
Sand
   Small granules of rock are more maleable than stone or metal but are less likely to hold their form when shaped.  Sand is ten times denser than water.  Sand has armour equal to half its Size and can be melted into glass by sufficient heat.

Stone
   Hard, natural material, ten times denser than water.   Stone is hard and rigid.  It has armour equal to its Size.  Stone does not burn, nor does it suffer damage from fire.

Metal
   While there are various metals, they all function much the same mechanically.  Steel has provides armour equal to its Size, with other metals providing proportional degrees of protection as shown on page pp .  Metal has a density ten times greater than water.  Fire that overcomes metal's armour can damage or even melt it if sufficient damage is inflicted.  Metal is conductive and any fire damage it takes is transferred directly to anyone in contact with it.

Water
   The base-line density for substances is one cubic metre equals one tonne, or in game terms one space equals Size 33.   Water provides a more focussed concentration of movement than air.

Wood
   Denser organic plant matter has twice the density of water.  It is a hard but flexible material with excellent durability and is used in most medieval technology.  Damaged structures and tools can be mended.  Such items can also be conjured.   Shaping wood is helpful in both crafting items and cultivating trees.