The Practical Use of Cerium Oxide
Cerium Oxide is a superabrasive powder used for polishing glass. It works by physical abrasion. It also chemically reacts with the elements in the glass. So it is a chemo-mechano polishing powder. Which is different from many other polishing powders such as silicon carbide, microcrystaline silica, or diamond. It has been used as a glass polish for many years. Especially in polishing plate glass (before float glass was developed), crystal glass, mirrors, and optics. Even though our application is similar in that we are also polishing flat glass, there is one major difference that few in the window cleaning industry have recognized. That is we are usually removing hard water deposits or mineral stains. Such mineral deposits interfere with the typical physics of the polishing process. Marc Tanner my polishing guru first brought this to my attention. Which means quite simply that the end results absolutely cannot be predicted. Every restoration job is different. It is also undeniable that the surface type will determine how easy it will be to remove the stain with a cerium oxide or any other polishing product. Dark tinted glass is the most difficult surface to clear/repair. Transparent glass is much easier. The low e coats are the most easy to clear.
What is the difference between suspended cerium and regular cerium? The short answer is nothing. Cerium oxide comes in either a plastic bag or a plastic bottle or pail. It comes as a totally dry powder. Suspended cerium is this same powder mixed in water with a special thickening chemical added. It is called a suspending agent and keeps the microscopic particles of the cerium powder evenly suspended in the water. If a suspending agent/thickener were not added to the water the cerium oxide would settle to the bottom of the water forming a thick mud that is very difficult to bring back into a homogeneous suspension. You wouldn't be able to just shake it. There are many different suspending agents but only two different types of suspensions. The first is permanent. The cerium will stay suspended for over a year. The second might only work for a half hour. Although in this case it is very easy just by shaking the container to bring the cerium back into suspension. It will not form a thick mud on the bottom. Then there is everything in between these two. If you need a suspension to last for the entire day or several hours instead of thirty minutes you will have to add more of the thickener. Again remember that the performance of the cerium does not depend on the suspension. Suspensions are mostly for convenience of use.
Now it is also true that the cerium to water ratio will also determine how much of the thickener you must add to attain the suspension you want. You can add 5 part cerium to 1 part water, or 1 part cerium to 5 parts water;...and everything in between. This would be the first step you should take. You will want a suspension/slurry with the least amount of cerium, but still one that will clear the glass effectively in the least amount of time. Only trial and error will reveal this to you. The chemist I consult with explained that he likes a ratio of 1 to 1. Kind of middle of the road. Once you have determined what ratio works best, you will need to add enough of your suspending agent to keep the cerium oxide suspended for the exact amount of time you will be using it. There are specialty chemists who can help you with this. It also is a good thing to keep a handheld battery operated pH meter around to check the pH of the solution. You will want a pH of 7 to 8. So a little on the alkaline side. Which can be attained by adding a little baking soda then checking with your meter. This also serves to minimize the problem of electrostatic agglomeration. This happens when the particles get electrically stuck together forming much larger particles that can easily scratch.
What is the best cerium oxide to use? You will want a cerium with an average particle size of 2 to 3 microns. I say average because no cerium powder has a precise particle size. All cerium powders have what is called a Particle Size Distribution curve or a PSD. To explain. A cerium that is rated as a two micron will have some particles that are only a half micron. It will also have particles that are one micron as we approach the two micron average. If your product has a very tight PSD then 98% or higher will be 2 microns. It will also have a very small percent of the particles up to 8 or 10 microns. This is called the tail end of the PSD curve. The tail is most important because it is always the larger particles that scratch. Yes it is true that all scratches begin with a PIF or Point Indent Fracture. Whether the particle that scratches is very small or very large. But the larger the particle the easier it is to see the scratch. So by choosing a somewhat smaller average particle size we minimize these larger particles in the tail and hence reduce the odds of leaving more scratches in the finished surface. By the way the tail is much more of a concern than electrostatic agglomeration which is generated more by the pH of the suspension. It is also true that the contamination of the cerium powder by other minerals is not as much of a problem as the tail. So people will talk about the pure white ceriums being the best while the yellows or orange ceriums are of a lesser quality. This is therefore not necessarily true. Once again the proof is in the tail which is defined by the PSD. Which you should be able to ask for in a tech sheet of the product. The tech sheet should also give you the percentage of the individual minerals in the product. Such as how much iron oxide or magnesium carbonate is in the cerium product. Iron oxide is the ingredient that gives the product its yellow or orange color. The bottom line here is to go with a 2 to 3 micron cerium and a very high purity such as a 99.9%. Then ask for the tech data sheet and look at the tail of the PSD along with the percentages of the minerals and metal oxides.
Not to be underestimated is the ability of certain acids to give you the cutting edge at removing mineral deposits that are based not so much on silicate chemistry. Organic acids such as citric or sulfamic do have the ability to chemically change the non-silicate minerals such as calcium and magnesium into soluble deposits. Once they have been converted they become much easier to remove by polishing with a cerium slurry.
There is another product that works with just water. The pad is loaded with a fine cerium and a touch of diamond. It works with pure soapy water. First you apply the water to a dirt free surface (but not stain free). Then you start polishing. The polishing particles are slowly released into the soapy water creating a slurry as you polish. There are variations of this type of product which have been created by other companies. They can be purchased and used with most commercial stain removal equipment available.
Another way to go is to first make up a rather thick glycerin compound (not a water slurry). This can be made at a four to one ration. Four parts cerium to one part glycerin by volume. You can use a motorized egg beater to slowly make your compound. Then use a felt polishing pad on a rotary polisher. Wet a clean window with soapy water. Apply around six to eight drops of compound to the felt pad. Then just go to the glass and polish. This process will help to control how much cerium you use by counting the drops you put on the pad. Since cerium is a very expensive product (although the price has come down in recent years) it is all about control. No matter whether you are using a slurry, compound, or a slow release pad.
Specific names of cerium products will be given in the Glass Smart Webinars. Samples will be available at the Fishkill Seminars. And videos of what I have explained here will shortly be available on my new YouTube Channel. Discussions will follow I am sure on the Glass Smart FB Group and other groups.
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Written by Henry Grover Jr.
henrygroverjr@gmail.com
