Characters in The Broken Hourglass have an almost unlimited ability to customize their spellbooks through the use of the Tinker Spell interface. A "tinkered" spell is one defined by the player, selecting from a menu of spell sources and effects and given a player-defined mana strength.

Tinkered spells may employ any number of the five elemental magic sources (Physical, Fire, Earth, Air, Water) and one or more of the following standard spell effects:

• Heal.
• Regenerate.
• Armor.
• Attribute Bonus.
• Damage.
• Repeating Damage.
• Life Drain.
• Lower Resistance.
• Attribute Penalty.
• Summon.

The Tinker Spell interface. This happens to be an Earth+Fire+Water Magic enchantment, causing Lower Resistance and Attribute Penalty effects, targeting all visible enemies.

The mana requirements for each spell effect are summed together, then multiplied by the target factor. In addition, a spell's base cost is multiplied by the number of distinct types of magic (e.g., Earth, Air) involved. Thus an Attribute Penalty spell with both Earth and Air costs twice as much mana as an Attribute Penalty spell with only Earth, but the first spell also inflicts twice as many penalties as the second one. Spells which affect an indiscriminate radius of targets are more expensive than those affecting just a single target, while spells which selectively affect all allies or all enemies have a still-higher multiplier. You may tinker a spell to use more mana than is strictly necessary, to create a spell with a greater potential effect.

Why tinker spells at all? Although your characters will start with a well-stocked spellbook containing many common and useful spells, there are so many different ways to combine these effects that we cannot possible account for them all. A character who specializes heavily in Air and Fire Magic may find it more beneficial to combine multiple damage sources into a single spell rather than alternating between airbolts and firebolts, for instance. Or such a character may wish to cast a single party-enhancing spell to boost strength and agility (from Fire and Air Magic, respectively) rather then split them up into separate casts. And as characters become more powerful, they may wish to devote more mana to a single spell than the default spells in the book specify.

- When a spell has multiple effects, the mana of the spell is split up among those different effects. So a 20-mana Fire Magic spell doing both attribute penalty and lower resistance will have essentially the same effect as a 10-mana attribute penalty spell and a 10-mana lower resistance spell. The advantage? The tinkered spell accomplishes both at the same time.

- It is not possible to tinker a spell which involves the unique Physical Magic effects--Heal, Regenerate, or Life Drain--along with a second elemental source. That is, you cannot have a spell which is Physical + Fire, life drain + direct damage. Those effects which are unique to physical magic must be cast as physical magic-only spells.

- It is possible, but inefficient and generally not recommended, to tinker a spell which involves Attribute Bonus or Attribute Penalty from one elemental source, plus a physical source. Physical magic does not have a concept of attribute alteration so that component of the spell is wasted and ignored, even though the caster will still commit the full mana charge to the spell.

- A caster's magic skill rating provides a bonus to the magic attack roll. When casting a tinkered spell with multiple elemental sources, that bonus is divided by the number of elements in the spell. For instance, if Tuhan has 50 points in Air Magic and 10 points in Water Magic, and casts an Air + Water Damage spell, his attack bonus from magic skills is (50 / 2) + (10 / 2) = 30. It would be a +50 bonus for an Air-only spell, so casters with disproportionate magical skills must weigh the costs and benefits before casting multi-source spells.