Surface Defects

Tin pick up: In the hot end area of the furnace (1100ºC), a relatively large amount of oxygen enters the tin bath as a result of a high oxygen diffusivity at elevated temperatures. As the liquid tin flows along with the glass sheet in the direction of the colder sections near the exit end (600-500ºC), the oxygen solubility decreases significantly. This results in the elimination of the oxygen from the tin melt in the form of tin dioxide (SnO₂) floating on the tin, or so-called dross.

Dross formed under the glass sheet, but also a tin melt with high oxygen content, may easily adhere to the bottom surface, usually referred to as tin pick up. Moreover, tin picked up from the bath via the bottom surface may adhere and solidify on the first few lift out rollers, causing even more damage to the bottom surface of the ribbon.

Bloom: The bottom surface defect bloom appears as a haze after post process heat treatments in air of the glass sheet such bending or tempering. It is the result of high tin levels in the bottom surface area of the glass sheet (high tin count values). Diffusion of tin from the bath into the bottom surface mainly occurs at the elevated temperatures in the hot end section of the bath. As most metals are usually insoluble in oxides, tin must first be oxidised to SnO before it may diffuse into the glass ribbon. Therefore, high oxygen levels in tin promote tin diffusion into the bottom surface area of the ribbon and thus promoting bloom formation during post process glass bending or tempering. A clear relation was found between oxygen content measured by a Read-Ox tin oxygen sensor in Bay 1 (hot end) and the analysed tin count values. Especially oxygen leaks in the bath perimeter (side wall sealing) in the hot end area should be eliminated to prevent bloom formation. For this purpose continous oxygen monitoring in the hot end is very important. When the oxygen sensor indicates an increasing oxygen level, the bath perimeter should be inspected and a sealing action may be considered. Moreover, the sensor provides a fast feedback on the effectiveness of a sealing action and critical areas may easily be identified.

Top surface defects such as cassiterite particles (SnO₂) and tin drops, are an indirect result of an oxidation process. Especially in the hot end section of the furnace, volatile SnO vaporizes from the tin bath and condenses on the colder parts of the superstructure and overhead equipment in the form of tin drops and cassiterite particles (as a result of a SnO-disproportionation reaction 2SnO -> Sn + SnO₂). These accumulated condensates will eventually come loose and drip onto the glass sheet, causing so-called top tin, top specks or even crater drip. Especially high hydrogen levels to reduce atmospheric oxygen (e.g. after a bath openings for maintenance actions), may cause the reduction of the tin oxide condensates on the superstructure, resulting in an increased dripping of tin onto the top surface as an unwanted side effect.

It is important to monitor the impact of bath openings on the bath' oxygen level, in order to prevent too much tin built-up. Additionally, bath emergencies such as a leaking tin cooler, releasing a large amount of water vapour (and thus oxygen!), should be detected as early as possible to limit the accumulation of large amounts of tin deposits on the colder roof sections. When using a conventional optical inspection system, detecting tin drops on the top surface, the leaking cooler will only be noticed when dripping has already started. In worst cases, dripping may continue for days loosing an enormous production volume! An on-line oxygen sensor however, immediately reacts to a sudden increasing oxygen level and the leaking cooler will be detected rapidly. As a result, the built-up of tin deposits is limited and consequently also the resulting tin dripping is limited or even prevented.