Homemade Silver Halide Plates
This is a remarkably simple way of making a holographic recording material. This is the way that we make a lot of holograms in our lab, and we find it to be by far the easiest method for making holographic plates.
In essence, we treat a glass surface to make it chemically 'sticky', we coat that with gelatin, and harden the gelatin with chromium or formaldehyde. Once we have the gelatin film we then soak into it a silver salt, and subsequently soak in potassium or lithium bromide to precipitate an ultra-fine grain precipitate of silver bromide. The bromide solution also incorporates a dye to make the plate photo-sensitive in the required wavelength range, and with addition of a little sensitiser we can produce by this method a holographic plate of quite high standard.
A worksheet (dated Nov 2000) is given below. It gives results which have good diffraction efficiency and photosensitivity compared to ultrafine grain proprietary material. But this is for the fun of doing it all yourself and getting bright results. If you are particularly concerned about marks from bubbles, dust and blemishes then you may prefer to use the proprietary material.
The material on this page is based on the following article, with some changes that we have made to our protocols since publication, and any differences between the original article and the text below are solely due to those differences.
- 1 A simple way to make silver halide hologram recording plates by Diffusion
A simple way to make silver halide hologram recording plates by Diffusion
By Jeff Blyth Institute of Biotechnology University of Cambridge Tennis Court Rd. Cambridge CB2 1QT
Tel: 01223 334152 (fax: 334162) email: firstname.lastname@example.org
What follows is in the form of a worksheet based on the paper published in The Imaging Science Journal, Vol. 47, pp 87-91, 1999. A text only version of the paper can be found on the Internet at http://www.holoworld.com/holo/paper.html or at http://www.holografie.com/paper.html.
The Basic principle
A coating of pure gelatin on a glass plate is treated with silver nitrate. The coating is then immersed in a bath of bromide ion and dye. This then precipitates extremely fine grains of silver bromide in the gelatin layer.
- Presubbed glass plates. You can use old holographic plates with the gelatin removed with the aid of household bleach.
- Gelatin of bloom strength between 250 and 300 (e.g. 300 bloom from Aldrich cat no. 27,162-4). You can use culinary gelatin without any sugar or flavourings.
- Ascorbic acid or Vitamin C.
- Silver-nitrate. A 1N volumetric standard solution is a useful form.
- Potassium Bromide
- Chromium acetate. You can use chrome alum instead.
- Pinacyanol Chloride for HeNe 633 nm exposure.
- 1,1-diethyl-2,2 cyanine iodide for 532 nm exposure.
- Sodium hydroxide
- 3-amino-propyltriethoxysilane (for new glass plates).
Concentration of Solutions
Quantities will need to be judged by you to suit your requirements.
- Silver nitrate: 6% w/v in (DI) water (or the 1N volumetric standard solution diluted by 1 volume to 2 volumes DI water).
- Stock dye solutions. (In practice, you would probably only need one hundredth of a gram to make up a few ml of these somewhat expensive dyes.)
- for 633 nm: 1 g / 1000 ml Methanol.
- for 532 nm: 1 g / 500 ml Methanol.
- Potassium bromide: 4% w/v in 3 / 2 methanol / water (3% lithium bromide gives a finer grained hologram than the equivalent concentration of potassium bromide, however 4% potassium bromide works well).
- Chromium acetate solution: 1% or Chrome Alum, 2%
- Gelatin solution: 15% (see 2 paragraphs down).
- Ascorbic acid: 1% solution in water, adjusted to around pH 5 with any alkali.
Preparation of plates
Glass plates usually need a pre-treatment step or the gelatin coating will peal off. You can use old holographic plates by simply giving them a 10 min. soak in neat domestic bleach solution and then rub off the old gelatin layer under tap water. After a final rinse in distilled water, no further subbing step may be required.
However with new glass plates, I leave them soaking overnight in a 100% bleach (Domestos or Parazone). After the plates are dry I rub them over with a 1% solution of 3-amino-propyltriethoxysilane in acetone on a tissue until it has evaporated, and leave them in air to interact with the silane for at least two hours before coating. (The silane solution has to be freshly prepared for each batch of plates).
Preparation of coating solution for a 10 x 8 plate
Add 30 g gelatin to 170 ml cold distilled water and mark the liquid level on the beaker. Place beaker in a water bath and heat while stirring constantly until gelatin solution is between 60 and 70o C.
Stir until all granules have cleared. Top up level to the mark. To remove skin and surface foam, pour through a fine mesh (nylon stocking works fine) into a preheated beaker. Then immediately proceed to next step:
Coating (by the old Victorian curtain method)
Hold the beaker in your right hand and with you left incline the presubbed glass plate (preheated to around 70oC) at an angle of about 30o to the vertical with its bottom edge in a clean tray. Pour the gelatin in a line about 1 cm from the top of the plate. The pouring rate must be continuous until the furthest edge of the plate is reached. (You may have to accept the tendency of the coating to not completely cover the lower part closest to the furthest edge.) Lean plate against something for a few minutes while coating gels. Run a knife along thick layer at the bottom to free plate rather than risk tearing the delicate coating. (Since no hardener is involved yet the gel can be readily scooped up and re-coated if you are not satisfied.). Put plate in cold solution of chromium acetate for 1 minute. Shake off drips and then (without washing away that salt) blow plate with cold air until dry. Once the layer is dry leave the plate to complete the chrome hardening effect overnight in a warmer. (Preferably at around 60oC for several hours). Rinse the hardened plate in DI water and dry in a warm air flow. If you want to cut plate up for the next step then after scoring the glass on the back and cracking it, it is best not to pull sections apart before running a scalpel blade along the gelatin side first so that it is cut and not torn apart.
Alternatively a Meyer bar can be used. About 7 turns per cm.
AgBr loading operation
- For a 5 x 4 plate place approx 3 ml 6% silver nitrate solution in the centre and at once squash it with a clean flat cover plate (preferably transparent plastic so that you can see the air bubbles are squeezed out). Leave for 3 minutes. Safelighting is not strictly necessary here but white lighting should be subdued.
- Remove cover plate and immediately remove the excess silver solution on its surface by gently brushing over the plate with a soft squeegee (windscreen wiper blade).
- Blow dry plate with cool air. Once dried, the plates can be stored for a short while in a cool, dry, dark location until needed.
- Under safelight conditions, add 2.5 ml of dye solution per 100 ml of potassium bromide solution, add about 0.5 ml of 1% ascorbic acid solution (this is the same solution as is used in the final sensitizing bath) agitate the bath and plunge plate in while maintaining the agitation for about 2 minutes (although with softer gelatin this could be reduced to 60 seconds, otherwise unacceptable grain growth can occur. Expect to spend a little time optimising this step for your own application). This solution can be re-used a number of times, until such a point as the dye starts to come out of solution or the brightness of the resultant holograms seems to be diminished; the dye used for 532nm exposures (see above) is far more re-usable.
- Rinse well under running tap water (any AgBr only on the surface can be removed by gently rubbing with ungloved finger.) Plates usually come out this bath beautifully clear under the green safe light, without any surface deposit.
- Sensitizing bath step
- The plate can be immersed for 1 minute in 1% ascorbic acid solution adjusted to pH 5 using a little sodium carbonate or hydroxide. Alternatively, the well known triethanolamine pre-swelling technique can be used with the advantage of increased brightness at a shorter wavelength. (Prolonged settling period may then be necessary however to avoid creep while the exposure is being made).
After exposure the plate is then developed as per the first part above.