Q. Why does glass need annealing??

A. To make it stable and prevent it breaking spontaneously. As the vials are formed the outer skin of the glass cools faster than its core, this affects the alignment of the molecules within the glass and creates tension and compression within the same piece of glass, this is known as strain and can be assessed using a strain gauge. A strain gauge uses polarised light to show the different refractive indices within a piece of glass, this causes the light to split into primary colours in the same manner as a prism. Tubular vials with their thin sidewall and thick thread exacerbate this situation as the wall cools more quickly than the thread. The solution is to anneal the glass, to do this we must heat it to above its strain point, at which point the molecules re-align, the glass is then cooled slowly rather than rapidly and the skin cools at a closer rate to the core and reduces the strain. Glass can be batch annealed in a kiln or can be heated on a moving belt through an oven known as a lehr.

Q. I have been told that storage conditions can cause deterioration of glass, is this true and does it apply to microscope slides?

A. Yes, all glass starts to deteriorate as soon as it has cooled! The type and rate of deterioration varies with the glass type, but in our industry we need only be concerned with soda glass. All soda glass, including microscope slides, suffers a deterioration to its surface, but if it is kept dry then the effect is rarely seen. If the glass is regularly washed, as in window or container glass then the effects can be removed. The reasons for this are in the active surface chemistry of the glass. Because of the high content of soda ash (Na₂O) in the melt the glass has free sodium ions available at its surface, calcium and potassium can also be available dependent on the glass type. If moisture is available then it can react with the sodium to form sodium hydroxide, this in turn can react with sulphur in the atmosphere to form sodium sulphate, this shows as a greasy bloom on the glass surface. The sulphates in particular can cause smearing and poor wettability on microscope slides. The moisture can be introduced by storing glass in damp conditions, or by allowing the storage temperature to rise and fall around the 'dew point' of the atmosphere, this results in condensation. At LSL we keep all our glassware in dry and heated storage areas to prevent such damage, microscope slides are stored in a dehumidified atmosphere to maximise their shelf life.