As the Zen iron ball that sits in the throat, going neither up nor down, alumina sits in a ceramic glaze stiffening and rigidifying it.

Soda lime glass, of which jelly jars are an example, can be crushed and placed on the inside of a ceramic bowl to great decorative effect. This glass will flow like water on a vertical surface.

Ceramic glazes, unlike bottles and other utilitarian glassware, glazes contain substantial amounts of alumina. This produces a viscous melt that flows little. The alumina stiffens the network. Analogous to the 12 ft long sofa that determines the position of every other piece of furniture in the living room, the alumina in the network constrains movement within. The visible effect in the silica-alumina network is to create tight strained connections.

This is illustrated below in two-dimensional cartoons that show the change in structure that accompanies successive additions of alumina to a glass melt.

In two dimensions, the silica is represented as a triangle. The network is built of these triangles, connected in pairs at their vertices. Alumina is represented as a square. It masquerades as a triangle, in that only 3 of its 4 vertices are connected. Three vertices of a square make a triangle, but a fat one. These fat alumina triangles are obstacles that obstruct the atomic configurations within the glass; much as rocks fallen into the road, or vehicles that aren't moving, redirect the movement of traffic. The orderly arangement of triangles forced around these oversized triangles is modified. Near an alumina the network is stretched and squeezed, resulting in a strained more rigid network.

Recall the structure of glassy silica in two dimensions - it is a network of triangles. A drunken network, not random, yet not the repeating tiles of a bathroom floor.

Observe the shape of the triangle 'in the alumina square', three of its four vertices are 'in the network', the remaining discarded. This triangle has two short sides and one long side.

The stretched shape of the alumina square acting as a triangle fits awkwardly into the network.

Note that with increased additions of alumina the network evolves into a structure of "connective tissue" (chains of triangles) holding the squares together.