Infusion is the process of binding items and essentia to a central item to change its properties. The process requires an Infusion altar, surrounded by two to twelve pedestals, which will hold the items to be infused. The items do not need to be placed in a particular order around the altar, however it is imperative to keep as much symmetry as possible.
This page gives a casual overview; for more detailed information, see Infusion mechanics.
In the Thaumonomicon, an instability difficulty is listed at the bottom of the infusion recipe, from negligible to dangerous. Instability can cause bad things to happen, such as items being knocked off of pedestals, or as far as releasing flux into the environment. For this reason, it's important to reduce instability by all means. Certain magical paraphernalia reduces the instability of the altar; however, the placement is important. Pedestals, ingredients, and stabilizers must be paired, across the latar, with 180-degree symmetry. (Same Y coordinate, but with opposite distances in X and Z.) Unmatched items will increase instability. Stabilizing items include mob or player heads/skulls, tallow candles, and crystal clusters. Stabilizers must be within a range of five blocks above the matrix, ten below, and 12 blocks in each horizontal direction. This allows plenty of space to place stabilizers, often in a "candle room" below the altar.
The picture on the right shows an infusion setup for creating a silverwood wand core. The first step is placing the target (a silverwood log) in the center of the altar, then placing the other ingredients on pedestals surrounding, maintaining symmetry as best as possible. Note the empty pedestal: Even with an odd number of ingredients, it's better to balance the pedestals. However, beginning the process now will only cause flux over time. Jars of essentia need to be present around the area (within the 25x25 box). Those do not have to be symmetrical. (More advanced thaumaturges can add essentia mirrors, to draw essentia from distant storage areas.) (For this recipe, 9 of each primal aspect and 9 Praecantatio are required.) The essentia must be in warded jars (or later, reservoirs), and during the process the essentia will be automatically drawn from the jars into the item. After the essentia is infused, the center item absorbs the surrounding items one by one, and if this succeeds, the center item will turn into the infused item.
"Arcane crafting and alchemy can go only so far. Something better is needed to create truly powerful mystical objects.
Infusion crafting is the process of infusing a single object with magical energy and the properties of other objects. For example, you could infuse a stone with vis and a feather and it would result in very light stone.
As is usual with Thaumaturgy, things are not quite that simple. To even get started with infusion crafting there are several things you need: a Runic Matrix, Arcane Pedestal, and a ready supply of magic in form of Essentia."
Later research will allow you to use the infusion altar for another use, enchanting. Using salis mundus, the appropriate item, and some essentia, you can apply any enchantment to an item. The rules for enchanting still apply; for example, you cannot apply both fire protection and blast protection to a piece of armor using infusion enchanting.