There I was assembling my camera gear for a trip to photograph the interiors of English cathedrals only to find a small crack in the ground glass of my 8"×10". Since I didn’t want to chance it, I tried to find a replacement. After an unsuccessful search, I decided to make my own. I remembered a recent article on view camera restoration by David Hoyt in which he mentioned using 280-400 grit carbide to make your own ground glass. I was familiar with lens grinding, having ground my own 8" reflective telescope mirror in high school. I even had on hand some of the grits packed away in the attic.
I visited my local glass supplier to find some glass similar to my cracked ground glass to use as a blank. I found out that the glass used in the familiar ground glass is 1/3 thinner than our single strength window glass. It was identified as “European glass” which isn’t imported to the States. Both had the same 70% silica and iron impurity content as evidenced by their identical green color. There was a corresponding difference in weight: my cracked ground glass weighed in at 7oz while the replacement US glass blank tipped the scale at 9.8oz. I reasoned that the extra thickness meant extra strength so I convinced myself that it was insurance that I could afford to live with; after all, an 8"×10" inch sheet of glass that is only about 2mm thick is easy to break.
To grind the blank, you will need a small ¼" thick sheet of glass to use as the grinding tool. Mine measured 4¼"×4¼". The exact size is not important but it should fit comfortably between your thumb and fingertips. It is a good idea to have both sheets of glass seamed to avoid cutting your hands while working with it. This is accomplished by running fine emery cloth across the outer edges. After laying out some newspaper, to your flat work surface, place the blank onto it. You need a small glass of water and teaspoon. You will also need another dry teaspoon to measure out your dry grinding grit. It is quite simple to do the grind: measure out about 1/8 teaspoon of grit onto the center of your blank with a similar amount of water and start grinding. You want to move your hand in a circular motion making about two inch circles while moving your hand in a large circle around the blank. In the case of an 8"×10" blank it will be necessary to also make a pass across the center. You will have to frequently add just a little more water. When you need to add water it will become obvious. The tool will be harder to move as the water evaporates and the two sheets of glass begin to stick together. Less frequently, you will need another charge of grit. The sound (as well as the thinning color) of the grinding slurry will be your prompt as to when you need to add more grinding material. With just a little practice this will all fall into place for you. You can see how you are doing by running a stream of water over the blank to flush all the grit from it and allow it to dry. You will see exactly where you need to spend more time grinding. It is finished when the dry sheet is uniformly white in appearance. How long it takes is dependent on the size of the grit you use and what the grit is made of.
I choose #380 grit as my starting point since it is on the finer side of the recommendation in Hoyt’s article. It took less than 10 minutes to complete but upon trying it out I found it to be too coarse. I clearly needed finer grit, which I obtained from Willmann-Bell, Inc. (phone: 800-825-7827). They sell optical grade abrasive at a very reasonable price. The woman at the other end of the line was most helpful and delivery was in three days. The next grade I tried was their #500 silicon carbide, which will yield a ground glass very similar to the one that came with your camera. By this time I was hooked and wanted to find out if I could make a better one than the original equipment.
One interesting thing that I discovered while using #380 or #500 grit is that chips of glass that are removed from the blank. This happens because these grits are made of silicon carbide, which is only slightly softer than diamonds. As you grind with these extremely sharp and hard grits, chips of glass flake out much like what happens to flint in arrow making. These chips, more than anything else, are what the problem is with normal ground glass. The chips are quite large and break up the image on the ground glass making it difficult to focus. If you look at your ground glass with a 22× loupe you can easily see them, as they appear dark surrounded by a white fine background. If it weren’t for these chips a ground glass would be much better. The problem is that these chips are very smooth and are at different angles, which cause the surface of the image to look speckled, that is to say, dark spots surrounded by light spots. The dark spots are these chips while the light spots are the ground surface of the glass. Because of this, when you use an 8× loupe, it is difficult to focus as they create a mosaic out of the image.
Now if instead you use white aluminum oxide you can avoid these chips entirely. This abrasive is much softer and super fine resulting in an incredibly fine smooth surface that is all white. It is almost invisible with a 22× loupe. The only problem is that it takes longer to make.
|ABRASIVE SIZE RELATIONSHIPS|
|Grit Type||Grit Size||Inches||Micro Size|
After studying the size relationship (see chart) I decided to try the white aluminum oxide micron grits. Several tries later, I have determined the best route is to start with a new blank and begin with the 5-micron powder: “dust” would be more descriptive. This will take about an hour. (It would not be a good idea to start with silicon carbide because you would have to grind out all the chips and these are surprisingly deep.) When you have ground for about 15 minutes you will see waves in the glass blank you are working on. Window glass, also known as float glass, is made by pouring molten glass on a layer of pure molten tin and often you will see wave patterns in the resulting glass. By the time you are finished, all of these waves will be ground flat. Clean both the tool and the ground glass very carefully several times under a stream of water to avoid contamination and do the final grind with 3 micron powder for another hour. This is the finest of the grinding compounds. It is the last grinding grit when making a telescope lens before the polishing stage. This is all worth the effort because when you are finished you will have a superior ground glass.
There are several unexpected advantages over original equipment. I set up my camera and temporarily placed the old ground glass on the camera’s back and focused the image and took meter readings using my digital Pentax spot meter under a dark cloth. Then I replaced it with the original ground glass to compare its readings. Both readings were made at the same five-inch distance from the image under the dark cloth. It turns out the new ground glass is twice as bright as the original ground glass. Cheaper and lighter than purchasing faster lenses! I repeated the measurements throughout the day, from bright sun to dusk, and the new glass was consistently a full stop to a stop and 1/3 brighter. So not only is it sharper and brighter but stronger as well.
I did notice one other slight advantage. If you use a fresnel lens, as I do, the groves of the fennel are less obvious because they are further from the image on the ground glass due to the extra 1mm thickness of the new glass. This is, however, only a slight improvement, as they don’t disappear entirely.
Vertical and horizontal lines can be drawn directly onto the ground surface with pencil. You can even make the lines clear by firmly attaching transparent tape to the ground surface. This will greatly reduce the light scatter caused by the ground surface and will appear darker, but you will still be able to focus. If you use tape, just be sure not to allow the tape to reach the edges of the glass, as that would space the ground surface one tape thickness further from the desired focal plane.
Before I leave the subject of broken ground glasses I found that a very adequate emergency replacement could be made. Just go to a local hardware store and have a glass cut to the proper size. Tape Scotch brand Magic tape to one surface of the glass. The tape is frosted and will work in a pinch as the ground surface. The image is serviceable but has a streaky nature to it. Make sure that the tape runs all the way to the edge so that the ”ground“ surface is at the proper plane.
You have your choice of spending 10 minutes to make a ground glass similar to what you could purchase or investing about two hours and owning one that has more desirable qualities. Users of 4×5 cameras may benefit the most from having a superior ground glass. Focus is more critical for smaller films because any focusing error will be magnified more in the enlargements necessary to make prints.