Devitrification and Solarization

Regarding Quartz

Regardless of the supplier, all fused quartz used in ICP glassware (no matter the manufacturer) begins life as sand, the majority of which is mined by the Unimin Corporation. In general, quartz tubing, rod and plate used in ICP and general labware are manufactured using one of two different manufacturing processes: electric fusion and flame fusion. Electric fusion involves the heating of the raw material (sand) in a electrically heated crucible perched atop a five story tower. As the molten quartz exits the crucible, it travels through a hole of a specific dimension and around a “pin”, also of a specific dimension, resulting in tubing of the desired dimension and tolerance. Most quartz used by Precision Glassblowing is manufactured using the electric fusion process. Flame fusion involves the introduction of sand into a hydrogen/oxygen flame. Upon introduction into the flame, the sand melts and collects on a rod which is slowly removed from the flame, resulting in a solid quartz ingot. Precision Glassblowing uses a high quality quartz that is produced using a three step process resulting in a quartz tubing with consistent dimensions and excellent clarity.

Quartz exists in a state of imbalance. The atomic structure of quartz glass is a chaotic maze of atoms and atomic bonds. Devitrification is simply the quartz atoms rearranging themselves into an orderly, crystalline structure. Quartz only exists as a smooth, clear material because, during the manufacturing process, the atoms have difficulty rearranging themselves in an orderly pattern before the molten material cools into a solid material. Devitirification is caused primarily by two factors: contamination and atmosphere. Contamination, as it relates to quartz devitrification, typically refers to the introduction of alkali to the quartz. Alkali’s include sodium, potassium and lithium and are usually introduced to the torch as a result of contact with the oils present in the skin. Fortunately, simply touching the quartz will not necessarily promote devitrification. A catalyst must be present to start the process. In the case of quartz, the catalyst is heat, specifically 1000 degrees C. It is interesting to note that contamination may also be introduced to the quartz by way of water. Water drops on the surface of the quartz may attract contaminants and, when dried, the contaminants may remain on the surface of the quartz.

Atmosphere also has an impact on devitrification. Quartz is manufactured in a partial vacuum and, as such, has an oxygen deficit. In order to re-crystalize, quartz requires a more perfect stoichiometry (a 1:2 ratio of silicon to oxygen in quartz). As the atmosphere contains a fair amount of oxygen, there is always oxygen present to promote devitrification, but adding additional oxygen will cause quartz to devitrify faster. Temperature also plays an obvious role in devitrification. As high temperatures are necessary to cause devitrification, the higher the temperature, the faster the quartz will devitrify. This explains why torches used in high temperature applications like oil labs tend to devitrify faster than those used in water labs.

Switching to a torch that incorporates different material for the outer shield is the ultimate fix for those applications most susceptible to solarization. Several OEMs offer an ‘organics’ torch which has an outer shield made from quartz that resists devitrification. This tubing is quite a bit more expensive than standard quartz tubing but if it extends the lifetime of the torch, it may be cheaper in the long run than using a standard torch. If you are running organics, check with your sales representative to see if your torch is available in a wear metals version.

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For users running organics at high power settings you will see a different form of devitrification called solarization. Again, this is a natural/normal phenomenon of glass. Although Precision uses quartz tubing with the highest purity available at >99.95% pure (total metal content published <20ppm by weight), there is up to 0.05% impurity. When this microscopic amount of impurity within the glass is subjected to deep UV (as would happen when running organics) in a high temperature environment (high power to run organics) the microscopic metallic impurities absorb the UV energy, causing localized devitrification at these specific sites. This devitrified glass has a different thermal coefficient of expansion than the surrounding glass and leads to microscopic ‘stress cracks’ as the torch is heated and cooled. This process is a natural and inevitable result when operating torches under the conditions inherent with organics. This evolution can be exacerbated by cleaning the torch. The small cracks, or broken bonds in the SiO2, created by solarization readily bond with the hydrogen component of most solutions used to clean torches such as hydrochloric acid, nitric acid, or even water greatly amplifying the internal stress caused by devitrification and increasing the onset of spider cracking and torch failure.