SUMMARY The construction and method of use of an inexpensive spray‐freezing device are described.

It is possible to sputter thin films of gold on to surfaces of frozen biological specimens at very low temperatures ( < 120 K) without untoward effects from heating. This is achieved by using permanent magnets to confine the plasma and thus to minimize the energy required to give a reasonable sputtering yield. The system described uses only 250 V at 12-15 mA to give 15 nm films within 2-3 min. It is shown, from theory portraying 'worst-case' conditions, that the specimen temperature could not increase by more than 6.0 K at equilibrium. Practical results support the theoretical assumptions. Similar considerations have been applied to sputtering at normal ambient temperature where it is shown that appropriate design of simple apparatus and selection of operational conditions can give adequate films in a reasonable time with negligible (< 3 K) temperature rise above the starting temperature.

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