[LargeFormat] Some Experiences with Re-cementing Lenses - part 1

[Richard Knoppow]

Some Experiences with Re-cementing Lenses



I recently re-cemented two lenses. There was a learning 
curve and I want to share what I found out.

The two lenses were a Kodak Ektar f/4.5, 152mm and a 
Wollensak Raptar Series 1a, 13", f/6.8

The Ektar is a Tessar type lens, the Raptar is a convertible 
Protar type lens.

There were several false starts on both lenses. Initially, I 
tried a two part heat curing cement. At the suggestion of a 
correspondent, John Hendry (pict at pict.co.uk)

I tried using an Ultra-Violet curing cement, I wound up 
using the latter. Both cements were obtained from Summers 
Optical.

John also suggested using Methylene Chloride to separate the 
lenses. I was a little sceptacle of this but tried it and 
found it superior to the high temperature solvent also sold 
by Summers.

Here are some of the things I found:

1, Clamping of the lens edges must be done using a machinist's 
V-block. The makeshift methods suggested on various web 
sites, such as using prisms, simply does not work reliably. 
Proper edge clamping is imperative for proper centering.

2, UV curing cement is much easier to use than the binary 
heat curing type, at least for small numbers of lenses.

The cement I used is Summers UV-74. This cement is one of 
the few that can be shipped as a non-hazardous material. The 
hazardous materials shipping on the other cements just about 
doubles their prices. UV-74 costs about $15 US for an ounce, 
enough to cement many lenses. Its shelf life is very long if 
stored in the refrigerator.

3, The official method for separating lenses cemented with 
synthetic cements is to boil them apart in a special solvent 
at 340 degrees F. It takes about five minutes for the lenses 
to come apart. However, its necessary to have a 
thermostatically controlled hot plate for this. I have a 
small laboratory hotplate but an electric frying pan can 
also be used.

There is a definite hazard of damaging the elements when 
using this method. At this high temperature the glass is 
very sensitive to shock. I got edge damage on nearly all the 
elements I cooked apart and will never use this method 
again. In some correspondence with John Hendry he suggested 
the use of Methylene chloride for de-cementing. I was at 
first a little spectacle but I obtained a good supply from 
Tri-S Sciences before they closed forever and tried it. The 
lens is simply immersed in the solvent for several days. I 
found that the UV cement would separate in about three days. 
This is all done at room temperature and there is no damage 
to the elements. I re-cemented both lenses several times 
while learning how to do it. The edge damage from the 
initial boiling apart does not seem to interfere with the 
lens performance.



I obtained Methylene Chloride from Tri-S Sciences before 
they closed but the same compound is available as paint 
stripper from the hardware store.

For older lenses cemented with Canada Balsam decementing is 
easily done with gentle heating. The traditional method is 
to heat the cemented elements on a hot plate but I've found 
that hot water works fine. Start with warm water and gently 
heat it until the elements slide apart. I don't know the 
temperature, and it probably varies, but its well below 
boiling.

Canada Balsam is cleaned with Acetone and alcohol as is 
described for synthetic cement below.

In the past I have tried recementing with Balsam but its 
actually more difficult than with a synthetic and is not a 
satisfactory cement.

Machinist's V-blocks are rather expensive new. I was lucky 
to find a used one, they seem to be very difficult to find 
used, but there is no substitute for one for reliably 
clamping the lenses.

Method of applying cement.

I followed pretty much the instructions on the Summers 
Optical web site. I also came on two books at the public 
library on lens manufacture published by Adam Hilger Co., an 
English maker of precision optical instruments. The methods 
described were about the same. Actually, the method does not 
differ substantially from that used for Canada Balsam.

Cleaning the elements.


I soaked the lenses in Acetone first. Then carefully wiped 
them and rinsed them in more Acetone. Then I cleaned them 
with Anhydrous Isopropyl Alcohol. This is obtainable at many 
drugstores and is not expensive. I then washed the elements 
in water with dishwashing detergent and rinsed them well. I 
used a final rinse of alcohol just before cementing. The 
room I used is reasonable dust free. Keeping the surfaces 
free of dust is very important. Probably the best solution 
if one has a dust problem is to build some sort of closed 
chamber with access holes for your arms and a window. I did 
not need anything this elaborate.

Small particles can be removed from the lens surfaces by 
using sticky tape, like Scotch tape, to pick them up. This 
tape does not seem to leave any residue.

I applied the cement with a wire made from a paper clip. Do 
not be stingy with the cement. Make a small pool in the cup 
of the concave surface. Then put the two elements together 
and press down gently to spread the cement out until it 
begins to leak out the edges. Air bubbles can be worked out 
by gently sliding the upper lens over the lower one. Slide 
them back and forth and vary the direction but do not use a 
rotary motion.

After as much excess cement comes out as can be expelled 
with gentle pressure, scrape off most of the excess with a 
razor blade or some other convenient tool Some excess should 
be left in place.

Now, place the lens, concave side down, in the V block and 
clamp it. I do not have a machinist's flat so I use a small 
sheet of plate glass. I clamp the V-block to the glass using 
a C-clamp. The clamp for the lens is part of the V-block. 
Inspect the lens to make sure its flat on the block and that 
the edges are aligned. Then apply the UV light for about a 
minute.

Summers UV-71 cures with an ordinary "black light" bulb. I 
have a type BLB but a BL should work as well. This is a 
tubular type lamp but there are now "compact" screw base UV 
lamps which will fit into a desk lamp or other convenient 
fixture.

After about one minute the cement will have