A method for partially freezing an aqueous mixture comprising simultaneously or in either order the steps of: placing said aqueous mixture in contact with at least part of a freezing surface; cooling the freezing surface to below the freezing point of the aqueous mixture; so that ice forms at the freezing surface; and oscillating the freezing surface relative to the aqueous mixture in a direction that is not perpendicular to at least part of the freezing surface; characterised in that the oscillation is linear with a frequency of between 20 and 200 Hz.