Media blasting is the process of blasting with abrasive media which was more commonly (and more correctly) called sand blasting. Back in the early days of blasting, the predominant abrasive of choice was sand. Silica sand is the more appropriate name – based on the fact that common sand is primarily comprised of mineral quartz, which itself is composed of silica, and oxygen. Silica sand was used because it’s cheap, readily available, and effective for many applications. These were simpler times when cost and results were given more weight than the health and safety of those exposed to it. Seat belts, for instance, weren’t required in cars and trucks. Cigarettes weren’t considered dangerous.
We now know that breathing silica dust is extremely dangerous. Silica sandblasting has long been closely associated with the disease historically referred to as miner’s phthisis, potter’s rot, or grinder’s asthma. Breathing “dust” in mining and other dust-producing activities has been associated with lung disease since ancient Greek and Roman times. More recently, with the industrial age, came inventions such as the pneumatic drill and jackhammer for boring into rock, (circa 1897), and sand blasting for finishing hard surfaces like iron and steel (circa 1904). These innovations produced large amounts of silica dust with little care, or understanding, for the health concerns they created. More accurately called silicosis, this deadly disease is caused by the inhalation of crystalline silica.
When considering abrasives, there are four very important physical characteristics that help to understand their use and application; 1) hardness, 2) shape, 3) friability, and 4) density. The first of these, hardness, is easily understood. We’re talking about mineral hardness. In this case, imagine the diamond scratching glass example that we’ve all seen demonstrated in one way or another. The diamond is minerally harder than the glass, when the edge of the diamond is scraped across the glass, it scratches. The explanation – diamond is harder than glass. When used as blast abrasives, harder materials are generally more “aggressive” than softer material.
Let’s examine some of today’s more popular blast media choices with these characteristics in mind as well as their effect on different applications. When you shift away from sand as a do-it-all blast medium, most people attempt to replace this dirt-cheap blast abrasive with something that offers increased value for the increase in cost. In other words, if you’re going to pay more for something, you look for a better, quicker, or more efficient medium for the additional money.
Baking Soda blasting
These abrasive blast particles have a knife-like shape that strips everything from light paint to heavy soot with relative ease. The density of baking soda is light, so much of it’s power comes from aggressive shape. The fact that baking soda abrasive is so non-damaging to substrates is due to its softness, and its friability. This blast abrasive pays back its higher price by offering several distinct benefits. It’s non-destructive to most materials including wood framing, and other building materials. Again, it’s friable and soft so it has virtually no impact on surfaces like steel, and glass. Here’s the real hidden benefit when blasted, the chemical side of baking soda also neutralizes odors from smoke and fire as well as mold, sewage, guano, drugs, and other smelly contaminants. This effect is multiplied by the friability factor – when media particles break easily, they create dust. Often wrongly seen as a negative, baking soda dust in contact with odor sources, means more effective odor reduction.
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Dry Ice media blasting
Dry ice is one of the most misunderstood blast media, but it can be examined in a similar fashion. Dry ice is solid carbon dioxide. As such it is “minerally soft”, and non-damaging to most hard surfaces like stainless steel, and other metals. The shape of most dry ice particles is rather “blocky” due to the particles being comprised of 1.5 mm diameter pellets. So, while not overly dense, it can still be somewhat aggressive against many coatings, and layers of debris. Blasting of industrial equipment and production lines are prime examples of where dry ice really shines is in the area of friability. Upon striking a hard surface, dry ice is smashed into such tiny pieces that the increased surface area of those particles can no longer remain solid at room temperature. They immediately sublimate (turning from a solid to a gas with no liquid state) and “disappear” into the surrounding atmosphere. Most often, this is highly advantageous, with the blasting process leaving no secondary waste – no spent medium. This leaves only primary waste – the dirt or coating – to be cleaned up and disposed of. One interesting note with dry ice is its extremely cold temperature when solid: -109°F below zero. This is both a positive and a negative when using this blast medium.
Walnut Shell and Corn Cob blasting
Walnut shell – and it’s agricultural cousin corn cob meal – are two of the more popular and affordable media classified as “blocky”. These two blast media are very similar in nature. Corn cob meal – the hard inner portion of the corncob – and walnut shell are both byproducts of their respective primary produce. As such, they’re inexpensive and generally available, with some variation due to seasonal and harvest outcomes. Their lack of friability might lead you to expect that they’re very aggressive against substrates. This is not entirely true, however, due to their very low density and mineral “softness” they remain nearly non-damaging. In fact, this lack of friability is one of the distinct advantages to these media – low dust levels from blasting. Their low cost, ability to effectively strip coatings, and their safe nature make them prime candidates for many large blasting jobs such warehouse fires that require quantities of one-pass blasting media. Please note that walnut shells are a tree nut. Operators should be screened for tree nut allergies before any exposure to walnut shells. When necessary, use corn cob meal as your primary choice and avoid the risk of an allergic reaction, which in some cases may be severe.
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Glass media blasting
First, let’s confront the fact that glass is primarily made from silica sand! This is where an understanding of organic chemistry (science) and physical properties of matter (engineering) would be helpful. We’ll just agree that while glass is made from silica, it’s not silica anymore. The basic mineral quartz has undergone a phase change and is no longer the crystalline structure that causes respiratory diseases like silicosis.
Adequate safety measures (respirators, ventilation, gloves, etc…) should still be taken, but the basic premise is that you’re protecting yourself from particulate that is a nuisance, not a killer. Crushed glass is sourced from recycled bottle glass and is readily available – thus a very inexpensive option as a blast abrasive. Crushed glass is angular in shape, and thus aggressive against coatings, and profiles (roughens) hard surfaces – beneficial when painting or powder-coating is necessary. Due to its low cost and ready availability, crushed glass is very popular when used in processes that only allow for one pass of the media through the system like wet abrasive blasting. Glass beads, on the other hand, are created by melting glass cullet, and spraying the molten glass and collecting the resulting droplets, or beads. This is a relatively expensive process, and glass beads are usually three to four times more expensive than crushed glass. Invaluable for applying a smooth, matte finish to stainless steel and aluminum, glass bead is a natural choice due primarily to it’s round shape.
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Plastic media
Plastic blasting media consists of soft, reusable plastic particles. It is ideal for a vast range of uses in the aerospace, automotive and electronics industries:
Plastic blasting is an environmentally safe alternative to chemical stripping. It includes none of the health issues associated with sand and hard abrasives.
