A catalyst carrier is judged on surface area and thermal shock. The honeycomb structure packs a large wall area into a small volume for the catalyst to sit on, while the open channels keep the pressure drop low, so the gas passes through freely. The very low thermal expansion of cordierite is what lets the carrier ride out the sharp temperature changes in a converter or an oxidiser without failing; the mullite and alumina types carry the higher heat, up to around 1700°C. The carriers are made as blocks and cylinders, in the cell count and size the catalyst and the reactor call for.