The kiln atmosphere can also have a strictly physical influence on the fired pieces. In many cases these physical effects are very marked.

A CO-containing atmosphere will for example increase the surface tension of the glazes with up to 25 % compared to an oxidizing kiln atmosphere. The significance of this is the observation of egg-shell surfaces in glazes. A CO-containing atmosphere gives smaller but more glaze-bubbles whereas oxidizing atmospheres generally results in fewer but larger bubbles.

Water-vapour in the kiln atmosphere will undoubtedly lead to a substantial lowering of the viscosity of glazes and liquid phases - hence give a noticeable lowering of the sintering- and/or maturing temperature of  most ceramic compositions. This decrease is the result of the breaking up of Si-O-Si bonds and the formation of Si-OH groups. The solubility of water-vapour in a glass-phase is proportional to the square root of the steampressure ⁽⁶⁾ . The solubility of H₂O in a glassy phase  at constant temperature increases with increasing alkalicontent and in the order Li, Na and K ⁽⁷⁾.

Light gases such as CH₄ and C₂H₄ in the kiln atmosphere will selectively be transported into the porous ceramic compositions. This transport is rather rapid, and once in the porestructure these gases can decompose and give carbon-trapping. The minimum temperature for carbon-decomposition is 850^oC for CH₄ and 550^o for C₂H₄. Clays, kaolins and colloidal substances will act as catalyst´s in this decomposition, and hence will be colored black on heating in a carbon-hydrogen containing atmosphere, whilst for example quartz, feldspars and (most) glazes will not blacken under these circumstances.