→Coating Curved glass surfaces such as wine glasses.
Martini glass with cut-off stem to afford possibility of constrcting a hologram with an all-round view
Wine-glasses have the very convenient property of being able to act as their own processing baths.It is best to do several glasses at the same time. I recommend getting 2 packs of 4 cheap plain wine glasses of about 250 ml (cc.) capacity. I first fill the glasses to the rim with either undiluted or 50% diluted domestic bleach solution.The bleach is left in each glass for about 15 minutes or longer. The bleach is then poured back into a bottle for re-use, and the glasses are rinsed well in running tap water and wiped around with a cloth before being given a rinse in DI. Each glass is then coated with the subbing layer described above under the title “Subbing the glass” but this time more conveniently, the subbing solution is poured into the glass and the glass is then tilted so that the soln wets the side of the glass up to the rim and the glass is then rotated so that the whole glass is wetted, the soln is then poured into the next warmed glass and so on. The treated glasses are then left standing in an airflow so that the very thin subbing film gets dry. The glasses are then placed in a preheated oven at 190-200C for about a half-hour. They are then allowed to cool and are given a rinse in DI.The glasses are then placed in a warmer at 50 C to dry. The bottle of stock gelatin solution should also be in the warmer and also a plastic or glass beaker with a spout.
A typical wine glass holds about 250 ml. of soln. So I take 100 ml (or it can be less) of the 50 oC gelatin stock solution prepared as previously described and I pour it carefully and slowly into the warm beaker while trying to avoid creating any bubbles. The beaker is then used to carefully pour the gelatin soln into the wine glass without creating bubbles, (the spout makes this requirement easier than pouring straight from the warm stock bottle). The wineglass is then tilted and rotated so that the inside is all wetted by the soln up to the rim of the glass. The gelatin is then poured slowly from the glass back into the beaker trying to again avoid creating bubbles. The glass is then upturned and placed on a clean surface allowing the gelatin solution to flow down to the rim before gelling at room temperature. Meanwhile the beaker of gelatin soln is poured into the next glass at 50 oC and so on. The soln in the beaker should not be allowed to drop below say 40 oC or it may get too viscous to coat properly. (I do not recommend using a few seconds in a microwave oven to reheat it as I find that it can damage the gelatin and cause fogging later).
Once the glass is cold and the gelatin film has gelled, then just the rim of each glass is carefully dipped into a bath of warm water to remove that thick gelatin layer around the rim. I try to not let this water level go higher than about 3 mm up from the rim. The glass is then still held in an upturned position and just the rim is wiped free of water droplets so that these droplets will not run down into the glass when it is placed upright. The upright glasses are then placed in a strong cool air current to remove the excess water in the gelatin. Once the gelatin film is unswollen (except for the partial swelling caused by the glycerol present), it is ready for the hardening solution to be poured in. The hardening soln is as described previously. Namely 4% chromium potassium sulfate soln with 4% glycerol chilled to about 5 oC. This solution is swirled around the wineglass so that it wets all the gelatin film and is then emptied into the next wineglass and so on. If the chromium soln has not run down the sides of the glass evenly when the glass is stood upright then any droplets should be gently removed by just touching with a paper tissue (do not wipe as the gelatin is very soft at this point), as only the absorbed chromium solution is wanted and droplets can cause a mark on the surface later. The wineglasses are then left upright in a cool airflow for several hours to harden. This hardening process is slow and if the room is particularly cold overnight say then it is best in the morning to warm the glass in a warmer at 50 oC for 20 minutes or more, or with a hairdryer at about 50 oC for a few minutes to finish off the hardening process.
Any excess chromium salt is then removed under cold running tap water. [Unlike the notorious dichromate which is (CrVI), it is OK to let a small amount of this form of chromium (CrIII) go down the drain]. The glass is then shaken free of tap water droplets and rinsed at least three times with DI.
Each glass in turn is then filled to the brim with a 0.1% solution of sodium carbonate for about 1 to 2 minutes and is then rinsed briefly in DI.
Now follows the vital step of removing traces of chloride ions from the gelatin film. Each glass is filled to the brim with about a 10% solution of potassium or sodium nitrate. They are then left for several hours for the ion exchange process to occur. (Cl exchanged for NO3). The nitrate solution is then poured back into a bottle where it is probably reusable several times more. The glasses are then rinsed in DI three times and left in a warm air-flow. When dry they are ready for loading up with the silver soln.
Diffusing in silver nitrate solution
For your first wine-glass experiments it is best to only expose one side of the wine glass and therefore you should only put your expensive silver solution on one side of the glass.
Covering the whole glass with silver causes real complications for an object such as a model figurine, because it will record a photographic shadow on the opposite side of the glass. It will also record spurious rainbow coloured effects due to transmission type gratings . However, it is worth covering the whole glass with silver solution if you want to just do a simple hologram of the glass filled to the brim with say coins. Then 2 exposures can be made 180 degrees apart and each exposure will not be affected by an exposure on the opposite side of the glass.
So for making a reflexion hologram on one side of the glass only, you need to first decide which half of the coating looks the best to use and then use a marker pen to put a cross on the other side that you do not want to use. To put the silver solution into one half of the gelatin film you need a new disposable rubber glove at least on one hand. Using a 1 ml clean plastic syringe take 0.5 ml of the silver nitrate solution, and holding the glass horizontally with the future image side down, gently empty the syringe onto that concave surface. Then use your gloved forefinger to wipe the silver solution only over that designated half of the glass. Spend at least 2 minutes gently wiping that solution over just that half of the glass. At first it might seem that the solution is not wetting the film surface well and is forming droplets. However as you work at rubbing it into the gelatin, the gelatin’s surfactant properties start to kick in and the silver soln spreads more easily. This operation can be done in subdued ambient lighting such as that from a 25W or 15W tungsten filament bulb, there is no need for a proper safelight at this stage but it is vulnerable to fluorescent or blue-rich lighting. Once the gelatin layer has absorbed the silver solution, it is important to remove any excess solution off the surface or it will scar the finished hologram. I use a folded tissue to gently wipe the gelatin surface free of solution. Only use the tissue for one single quick wipe then throw it away and use another clean tissue if needed. If you plan to use the whole glass for 2 images on opposite sides then you will need to use 1.0 ml of silver nitrate soln and spend at least 3 minutes rubbing the solution over the whole surface before wiping off the excess with tissues.
The glass should now look clear and scatter-free at this point, a hairdryer is now needed to dry it in a tepid airflow because it must be fairly dry for the next step, but a hot blow must not be used.
Sensitizing the gelatin film.Suitable safe lighting is now needed for the next steps. The average wine-glass will take 250 ml of liquid.So it can be convenient to pour say 300 ml of the stock sodium chloride (section 18 above) into a beaker and add about 6ml of the stock dye solution (section 19). Then about 250 ml of it can be rapidly poured into the wineglass and left in for only 45 seconds and then quickly poured back into the beaker and the glass then put into running tap water with minimal delay. A beaker should be used each time for each glass rather than pouring from one glass to the next. The timing here is important because if it is too short , not all the silver nitrate in the gelatin will be able to be converted to silver chloride and if it is too long there will be quite rapid grain growth. The thickness of the gelatin layer is also a factor in determining the optimum time. To stop the reaction of the sodium chloride solution, the glass is filled rapidly with running tap water and left in the water for several minutes. It is then shaken free of tap water and given a brief rinse in de-ionized water and then filled with about 3% vitamin C solution that has been taken to a pH of between 4.5 and 5 with sodium hydroxide or with sodium carbonate (a pH of over 7 acts as a developer and will cause chemical fogging) the glass is then given a brief DI rinse and left to dry in a good cool air flow. If you wish to use a red laser to make bright yellow-green holograms then the ascorbic acid treatment should not be used and should be substituted with a soln of 12% TEA. After a minute or two in this TEA solution, the gelatin surface should be gently wiped with tissues so that no droplets or rivulets remain. The outside of the glass should be wiped too of course so that it is smear free. The photosensitive glasses should then be dried in a good air flow and left to equilibrate in a light-tight cardboard box or cupboard. They should not be used till at least a day has passed because the gelatin layer will probably be quite unstable for many hours.
It is most convenient to have a 2 part developer in two one litre bottles labelled A and B. The developer can keep indefinitely in these separate bottles. They are then used by mixing equal volumes.
The developer below has been christened “TJ1” developer. (In honor of the late Tung Jeong who asked me to make a suitable developer some years ago for teaching his students).
Part A6g Metol (4-methylaminophenol sulfate)1 litre deionized water
Dissolve up first then add:
(Without the Metol, the developer will still work but is slower to act and the results may be less good.)
100g sodium carbonate anhydrous30g sodium hydroxide1 litre deionized water.
(This one should be labelled "very caustic" use rubber gloves and eye protection --guard against splashing it around.)
Just use equal volumes of A and B. For the 8 cm square flat glass plates, use the "floating dish" method. Two close-fitting plastic dishes are arranged so that one floats on top of the other. The volume in the lower dish should be just enough (50-60 ml) to give a minimal air gap so that the uptake of oxygen is minimised and the top dish can be used as a rocker to agitate developer over a plate.
Developer TJ1 is intended to react quickly (to keep the silver grains spheroidal rather than filamentary, and to minimize damage to the gelatin in the strongly alkaline solution). So sufficient exposure level to give a development time of only 15-30 seconds should be aimed for. To judge when the time is enough if you have not had previous experience is a little tricky because it is rather subjective. Here is my method for beginners:-
Take a small sample of the holographic recording material you will be using and totally expose it to bright daylight for at least 5 minutes. Then put it in developer for a minute. Then wash it well in a bowl of tap water and dry it . Now keep that piece of plate or film as a reference for your dark room. With your eyes adapted to your darkroom safelight, shut one eye and look through that piece of reference plate at your safelight.( Holographic recording material is never completely opaque). The degree of transparency or optical density of your reference plate tells you that the darkest parts of your developing hologram must never get as dark as that reference plate as judged with one eye shut. I just go for a “half as dark” subjective estimate, while holding the developing plate up to see the safelight through it. I then plunge plate into a dish under running tap water and then into a “stop bath” of 5% acetic acid, or straight into a dish of bleach solution which also contains a lot of acetic acid and oxidant which rapidly stops the development process in the plate. Ordinary room lighting can then be switched on.
It is best to first wet your plate in DI before putting it in developer because it helps to even out the development over the plate area when the development is going to be rapid. If the plate in the white dish of developer appears to go black in only about 5 seconds then that means the development should be brought to a halt within about 15 seconds (allowing for that very rapid one eye inspection of the safelight). In that case do not carry on developing for a minute just because that is a more conventional developer time. More often than not, a rapid development and rapid stop in a bleach bath give the brightest results. However, if the film only starts to darken after about 15 seconds then about a 1 minute development time is probably about right in TJ1 developer but it may still need to be longer. Ideally the laser exposure times need to be sufficient to give a development time of not more than half a minute at 22C . A room temperature below 20C will however slow the whole process down. The development is stopped by first rapidly plunging plate into a big tap water bath for a second and then into a bath of the recommended bleach soln.
The developer's useful lifetime with the floating dish method can be days, depending on usage. A yellow or mild brown colour means the developer is still good. When the developer is very dark brown it should be discarded. (It is the Metol constituent that is causing the brown colour when it is oxidised and it acts as a helpful indicator of exhaustion).
For reflection holograms, I find the most easily obtainable bleach is made as follows:-
20g. Copper sulphate (CuSO4. 5H2O)80g. Potassium bromide (or sodium bromide)70 ml Acetic acidDI to 1 litre.
This is known as a rehalogenating bleach. (This is not the best for transmission holograms however). The bleach does promote grain growth, so the time in the bleach should be minimized. The bleach process should not be allowed to take much longer than about a minute. If there is a very dark area (due to over-developed up silver) that has not gone after about a minute and a half , it is best to stop the bleaching reaction anyway and wash the plate under running tap water . That still dark part will clear in the final bath used next. As this bath does not contain a lot of bromide ions there is no tendency for it to encourage more grain growth.
After rinsing well under tap water, a final bath to prevent “Print Out” is needed.
Anti-Printout stock solution“Print-out”
Is an old term used in photography . It means a darkening occurs in the finished photograph (hologram) in ambient lighting , particularly in sunshine.40 g. Sodium persulfate (or ammonium or potassium persulfate)40g. Sodium hydrogen sulfateDI to 1 litre.
3 minutes in this bath followed by a very brief rinse in DI gives good print-out resistance. (Always make sure that your final rinse water is free from any traces of developer ).
Bleach for transmission holograms.
The anti-printout solution above will also work as a bleach for transmission holograms. This type of bleach is known as a “solvent bleach” which means that all the developed-up silver goes into solution and the final hologram will be made only from the original silver halide in the dark fringes that did not develop up, leaving the light fringes as just gelatin. It is very important in this type of bleach to have the developed-up plate well washed in DI before going into the bleach bath. This is because the plate has to be free of soluble bromide salt as does the bath itself. Then after the bleaching process is complete the plate should again first be rinsed in DI water before being given a final tap water rinse. The reason for this requirement is as follows. A final hologram’s diffractive efficiency relies on the refractive index difference between its light and dark fringes i.e. its fringe contrast. In the solvent bleach system the fringe contrast is got from silver halide left in the dark fringes against just gelatin in the now emptied-out light fringes. The removed dark silver metal is now ideally all in solution as soluble silver sulfate. However if any soluble halide ions were still present in the gelatin then these will combine at once with the developed-up silver as it is being oxidized to silver sulfate and the silver will be deposited back in the light fringes as silver halide where it will spoil the fringe contrast because the light fringes will not be just emptied out gelatin. In case you are wondering where soluble halide ions in the gelatin have come from, the answer is that they are always going to be produced by the developer---as the silver bromide gets converted to dark silver metal and the bromide goes into solution as sodium bromide salt.
The set-up for exposing the horse hologram is shown here
Ideally one needs to use a laser with a good coherence length. What this means in simple terms for making holograms using one beam is as follows. The diverged laser beam has to pass through the recording plate and illuminate a stable object on the other side of the plate, and then the light from the object has to pass back through the plate and form standing waves with the incoming light. These are recorded as “fringes” running inside the gelatin like the pages of a book. This forms a reflection hologram also known as a Denisyuk hologram. If the laser light were made of just one single wavelength that would be perfect because the standing wave pattern would be continuous from the plate to the object regardless of how far away the object was, provided that enough light comes back from it to form recordable light and dark fringes. However lasers are rarely so perfect and the light is usually made of a very narrow spread of wavelengths. If the object is too far back then the returning light gets out of step with the incoming light and instead of the crests and troughs of the waves coinciding they clash and fail to form the standing wave pattern. (But, at a still greater distance they do come back into step again). From the earliest days of holography, the main workhorse laser was the Helium-Neon or HeNe laser. This can give a depth of over 20 cm. in a hologram using the red 632.8 nm wavelength. However in recent years compared to the cost of a HeNe laser, incredibly cheap red laser pointers have become available and I have found that in the case of the cheapest small ones (~5 mw) I can get a surprising depth of several cm. operating in the correct mode. (A wrong mode causes striped patterns in the hologram but this effect can be temporary). The much more powerful red laser pointers with powers around 50 and 100mw., I found quite useless for holography unfortunately.