When it comes to kosherization, the distinction between steel and glass is a significant factor. In fact, the kosherization of what was reportedly a stainless-steel reactor sometimes must be delayed or canceled because the OU Kosher field representative supervising the kosherization discovers that the interior of the reactor was not only metal but coated with a thin layer of glass.
Stainless steel, a chromium alloy, is metal and can be kosherized. Glass is much more complicated. OU Kosher’s policy characterizes glass as a composite material (similar in this respect to ceramic). As a result, OU Kosher will generally not kosherize a piece of glass equipment. This is particularly the case regarding a kosherization intended to prepare a piece of equipment for a Passover production; the OU will not allow kosherization under such circumstances.
Since the glass coating is on the interior of the vessel, it is easy to make this mistake. This type of vessel – whether it’s a reactor, a pump, or other piece of equipment – is likely found in the production of pharmaceutical products, which have low tolerance for heavy metal contamination. An OU company that blends pharmaceutical glaze, better known as shellac, with ethanol, uses a glass-lined reactor. The processor requested that the same reactor also be used to produce dairy products, which would necessitate koshering the reactor periodically (in this case the unique design of the reactor permitted koshering).
Glass-lined vessels are not limited to pharmaceuticals. Recently a manufacturer of ethyl maltol, a chemical that provides a sweet-caramel note prized in the flavor industry, requested that the OU supervise a special Pesach production. One of the raw materials used to make ethyl maltol is furfural. When evaluating the viability of koshering, the mashgiach learned that every vessel used to process furfural was glass-lined (whether this was because of the corrosiveness of furfural on stainless steel or for other reasons, it’s not clear). These glass-lined vessels became the bottleneck to the project.
When developing a strategy for kosherization a glass-lined vessel it is critical to determine whether the interior is metal or glass (or some other material). The engineer or operator should be aware of which vessels have glass-lining. If the engineer is not available, one way to independently verify the materials used in a piece of equipment is simply to note the brand, model, and make of the vessel. The manufacturer’s website should have technical information relevant to the product. Finally, when safe, practical and legal, one can simply stick one’s head inside the equipment to determine the equipment’s interior material.