Doublet Cemented Lenses

Before you do anything, please read everything I have written carefully.

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How to make your lens elements really clean?

Canadian balsam solution

How to know if your flint & crown are in line?

Do not place the paper sleeve back on them. It will suck balsam out of your joint just like a wick.

40 or 50 years ago, Canadian balsam and clear rosin were available at hardware stores, drug stores and even general grocers. At that time, I lived in a small town near the Mexican border, and I had no problem buying either of these products very inexpensively. But things change, don't they? Today it may be difficult to locate many products which were commonly used even 20 years ago.Well, let's look at Canada balsam. You can still purchase it, but you must know where to look. (hint: The Surplus Shed) A small amount will last you many years. I am still using some I purchased around , and because it is almost gone, I purchased some from the Surplus Shed a few years ago.Often you will find a lens which looks cloudy or which seems to have many tiny white spots imbedded inside one of the elements. Some people think this is fungus, but if the problem is a cemented doublet, the problem may simply be trouble in the cement. In my experience, fungus does not live in Canadian balsam. An achromatic doublet is 2 pieces of glass used in combination to produce fewer color defects. In almost every case, one element will be flint glass and one will be crown glass. Both glasses have different refractive qualities.Like any process, there are many ways this can be done, and other people may use methods which seem better to them or which are more comfortable to their brains. This is simply how I have learned do it...A typical cemented doublet with problems:You can see it is quite foggy with white specks that look like falling snow. It also has an odd center spot that looks like a contact lens. Below you see I have taped stiff paper around it to secure the two elements. In order to repair the lens, we must separate these elements first, and since they are going to get hot in the process, I don't want to drop one:The lens element is now placed on top of my light bulb. It is a 75 watt bulb, and I think either a 60 watt or 100 watt will work also. Leave the doublet on the light bulb for about 10 minutes, remove it using a glove (it will be very hot), push it out of the paper collar and slide the top(crown) element off of the flint element. If you got the doublet hot enough, it should separate quickly and easily:In this case, the flint is darker than the crown. Since this is an old doublet, the flint (concave) probably contains lead. Make a note to yourself about how the two elements fit together. Now clean both elements completely with xylene (xylol), acetone, 100% alcohol or mineral spirits. Clean every bit of old glue from them, and be sure to clean off any lint or dust.After you clean all the old balsam off your elements, they should be very clear and clean-looking. If you can not make them clear, you can improve them with a non-abrasive polish sold as FLITZ paste. Rub them with a bit of this paste (not too much) and polish with a soft cloth.raw Canada balsam:dissolved in xylene (xylol) equals:Balsam is similar to rosin, but not identical. In the absence of balsam you can use high grade rosin. You can also use balsam which has been prepared for microscope slide use. There are alternatives: you can use clear epoxy (but I do not like it at all). You can use UV cured cement. It is ok, but it is very expensive and must be used in 4 months or it is no good. Also, you can not easily separate the elements later. If you are going to be working on many old lenses, get some balsam and learn the technique. You will be happy you did. How to make the solution: slowly dissolve balsam in xylene until no more will dissolve. The solution will become relatively the same color as the raw balsam. Store it in a tiny jar. It will never go bad.Ok, now I will place about 10 drops of balsam solution in the flint element:and then I will set the crown element in place. It is better to have too much cement than too little. It may run out of the seam, but this is ok. Remember, we will clean the excess off with solvent after the doublet has dried.Use your eyes, your sense of touch and your tongue. Your tongue is capable of sensing very tiny differences in height. When you have the elements in line, set them aside.Leave your doublet undisturbed in a quiet and warm place for a minimum of 1 week and maybe as long as 1 month. A sunny window is good, but do not let your doublet reach a temperature of higher than about 90 degrees F or the solvent in the solution will bubble (a disaster for you as bubbles were probably the problem you were repairing in the first place) Balsam around the edge will dry rather quickly, however the balsam in the very center will take possibly years to dry...and it is possible that it may never dry completely I suppose.Ok, after your doublet has dried for a week to a month, clean the edges with solvent very carefully (do not allow solvent to go into the seam). Obviously you do not want to disturb the seam because this is where your balsam will be dry.Now you will notice you have removed the black coloring from around the perimeter of your doublet (if it existed originally). To replace it, use a black felt tip permanent marker (I prefer the brand "Sharpie"). To avoid coloring the glass, place your finger in the depression as you see me doing:Remember, it is easier to clean your fingertip than to clean ink off of your glass.Reassemble the elements in your lens and enjoy it.Jon Goodman, , all rights reserved under US and international copyright laws.