Surface Defects

Tin pick up: In the hot end area (1000 - 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 travels with the glass ribbon towards the cooler zones near the exit (600-500ºC), oxygen solubility drops sharply. This triggers oxygen release from the tin melt, forming tin dioxide SnO₂ (commonly known as dross), which floats on the tin surface.

Dross accumulation beneath the glass, but  also tin melt with high oxygen content,  easily adheres to the ribbon's bottom side, usually referred to as tin pick up. Additionally, tin transferred from the bath to the bottom surface may easily adhere and solidify on the initial water-cooled steel lift out rollers, causing mechanical damage to the bottom surface.

Bloom Formation: Bloom manifests as a hazy appearance on the ribbon’s underside following post-process heat treatments such as bending or tempering. It stems from elevated tin concentrations in the bottom surface layer. Tin diffusion from the bath into the glass occurs primarily at high temperatures in the hot end. Since metals are generally insoluble in oxides, tin must first oxidize to SnO before diffusing into the ribbon. Elevated oxygen levels in the tin bath facilitate this oxidation, thereby increasing tin diffusion and promoting bloom during subsequent thermal processing. A strong correlation has been observed between oxygen levels measured by the Read-Ox tin oxygen sensor in Bay 1 and the resulting tin count values. To mitigate bloom formation, it is critical to eliminate oxygen ingress, particularly via side wall seal leaks in the hot end. Continuous oxygen monitoring is essential. When rising oxygen levels are detected, immediate inspection and sealing interventions are advised. The sensor also enables rapid feedback on sealing effectiveness and helps pinpoint critical leak zones.

Top surface defects such as cassiterite particles (SnO₂) and tin droplets, originate indirectly from oxidation processes within the tin bath.  In the hot end zone, volatile tin monoxide (SnO) evaporates from the tin bath and condenses onto cooler areas of the superstructure and (water-cooled) overhead equipment. This condensation leads to the formation of tin drops and cassiterite via SnO disproportionation (2SnO → Sn + SnO₂). Over time, these deposits grow and finally detach and fall onto the glass ribbon, resulting in surface flaws such as top tin, specks, or crater drip. Elevated hydrogen levels, often introduced to reduce atmospheric oxygen after maintenance-related bath openings, can unintentionally reduce tin oxide condensates on the superstructure. This accelerates tin dripping onto the ribbon’s top surface, a highly undesirable side effect.

Controlling oxygen levels in the tin bath is critical, especially following bath openings, to prevent excessive tin build-up. Emergencies such as a leaking tin cooler can release substantial amounts of water vapor (and thus oxygen), accelerating tin condensation on colder roof sections. Conventional optical inspection systems only detect tin drops once dripping has begun, often too late. In severe cases, dripping may persist for days, resulting in significant production losses. In contrast, an on-line oxygen sensor provides immediate feedback by detecting sudden oxygen increase, enabling rapid identification of leaks. Early intervention limits tin accumulation and effectively reduces, or even prevents, subsequent tin dripping from overhead structures.