It is especially important to shield the outside edges of the ribbon from heat so that they are firmer and will hold the rest of the ribbon in a proper shape. In some cases manufacturers will allow the edges to form thicker "bulbs", which are removed after final cutting. Immediately after being drawn the ribbon is cooled using mechanical coolers so that it maintains its rectangular shape in two dimensions, but assumes a ribbon like structure that extends down into the Debi and upwards into a drawing assembly. This mechanical cooling allows the ribbon to hold its integrity. In the author's experience the mechanical coolers used water, contained in specially shaped radiators, to remove heat radiated by the ribbon. Sometimes a mild vacuum is applied to the ribbon in this early part of the process since mechanical cooling can induce air currents which impact upon surface quality. Some manufacturers also will apply sulfur dioxide gas during the draw in order to change the chemistry of the glass on the surface. By changing the chemistry it is possible to affect the surface characteristics of the glass, improving its quality and durability. Glass rollers hold the ribbon throughout various parts of the process, supporting its weight and continuing the drawing process. The process continues as the ribbon is drawn upwards into a chimney like structure, where it is quenched or rapidly cooled. When the ribbon reaches the end of the process it is scored, or cut, and then removed for further processing in discrete sheets of flat glass. The "bulb edges" are recycled as cullet (flawed glass which is remelted) or were resold for shelving or displays. Sometimes flawed parts of the sheets were removed, leaving behind decent quality flat glass. Time, speed and spacing of the various phases of the process are critical factors in the Fourcault process. Fourcault process machine operators require experience in order to judge placement of the die, location of various parts of the process, and rates of draw. These must be balanced against glass quality and the age of the draw. As the draw continues the glass in the pit grows cooler and cooler, eventually leading to failures or diminished quality. The draw must be stopped, the pit must be "heated back" and then the process can continue anew. Glass chemistry has a huge impact on the process since it controls the melting, forming and annealing temperatures, liquidus temperature (point where various chemicals that make up the glass start to crystallize out of the glass) and rates of change of characteristics of the glass itself. Occasionally the ribbon will break or crack, leading to failure of the drawing process.