In a wet-gate, the original passes between two optical pieces of glass, between which the liquid circulates. The technique was perfected by a Dr Ott and for many years, any wet printing gate was known as an Ott Gate and used almost exclusively for optical step printers. The greatest problem with a wet gate of this type is that the liquid is at risk of leaking out where the film passes in and out of the gate, as no seals are ever fully leak proof. As the film passes through the seals, it is inevitable that air bubbles will be pulled in and trapped in the gate. If this happens, the bubbles can be seen as images on the print! The earliest gates all suffered from this problem, which was eventually overcome by pumping the fluid, round a circuit that included the gate, a reservoir of fluid, and a filter [to trap dust and dirt]. The pumping action pressurised the fluid, which prevented air bubbles from entering. The early Ott gates all leaked especially when a splice went through the seals; indeed the early wet gates were called "splash gates" in some British laboratories! Modern wet gates as used on Oxberry, Debrie, and Neilson-Hordell and the Peterson, Schmitzer and other modern wet gates used on machines are very different and the improvements are largely due to the choice of modern materials for the seals and pipes and pumps. Viton and Neoprene have revolutionised wet printing today. Ott type gates all suffer from the problems of bubbles of air trapped in the gate, especially at the start of a printing run, and these are overcome by pumping the fluid through the gate and through a liquid chamber that can trap or eliminate bubbles.
The Ott type wet gate can be applied more easily to optical printing than to contact printing.
Archival shrunken films are difficult to use on Ott gates, because the presence of the liquid and two tight seals does not easily allow the slip between the two films. This problem was overcome in the past by the use of lubricants [simple paraffin wax] mixed into the liquid or by using a greater amount of the liquid itself as a lubricant and pre-wetting the films. However other, better methods exist today.