A proper rinse is very important for making good holograms. There are a few things to consider. The first is the length of the rinse. A good rinse is long enough to remove all of active chemicals. 5 minutes is often used.
Water is more than just H2O. Tap water starts as rain fall. This water filters down through the ground to an aquifer. From the aquifer it either comes up through a spring or is pumped up through a well. This path disolves many things into the water. Gypsom, Epsom, Calcium Chloride and salt are often encountered. Also, it is common for water to disolve traces of iron on the way to the tap.
Ions in Tap Water:
- Ca++ Calcium
- Mg++ Magnesium
- Na+ Sodium
- Cl- Cloride
- SO4-- Sulfate
- HCO3- Hydrogen Carbonate and to a lesser extent H2CO3 and CO3--.
It can also contain disolved CO2 as carbonic acid.
Tap water rinsing
By Jeff Blyth
Prolonged tap water rinsing can remove some of your AgBr with significant differences depending on time of year and the temperature of your cold water supply. Any AgBr loss causes a shift to a shorter wavelength replay in the case of reflection holograms and of course some loss in diffraction efficiency but sometimes people prefer to simply shift the color from orange-yellow to yellow-green using a hot water rinse. The result can look brighter, also any scatter from AgCl contamination can be removed because AgCl is about ten times more soluble than AgBr.
Some idea of the temperature effect can be seen from this graph:
Importance of De-ionized Rinsing Water when "reversal" bleach is used.
By Jeff Blyth
If you are using a rehalogenating bleach then tap water alone is OK but with reversal bleach it is really important to avoid soluble halide ions (ie. bromide,chloride or iodide ions) remaining in your hologram before it goes into the dichromate bleach bath. So the procedure should be: After the developer the hologram needs a good rinse under tap water to remove the developer and soluble bromide and iodide ions in it . Even if the developer had no halide ions initially, the development process means that the AgBr and AgI in the emulsion had to be broken up and turned into dark silver and soluble Br- and I-. The tap water rinse then leaves the emulsion with just chloride ions from tap water which are less of a problem to deal with later than soluble bromide or iodide ions.
Before the dichromate bath is used you have to have two pre-baths of de-ionized water (DI) to remove all traces of dissolved halide ions. If you don’t do this then some of the developed up silver fails to be removed from the light-struck fringes and deposits itself back in the fringe as silver halide. This causes scatter in the finished hologram and reduces diffraction efficiency because the light struck fringes have failed to be properly cleared of AgBr. Where even experienced holographers commonly go wrong is that after removing the bleached hologram from this reversal bleach bath, they rinse it under the tap instead of first putting the hologram back in de-ionized water for a second time.
This is because after leaving the bleach bath the hologram is full of silver ions in solution which can instantly form silver chloride particles with the chloride ions in tap water. So this causes scattering from inside the emulsion which cannot be wiped away even if surface silver chloride can be.
After using the bleach bath you may notice a red-brown precipitate or scum in the bath. This is normal and it is actually good to have it in there. It is made up of silver chromate or dichromate which is not very soluble but is far more soluble than are the silver halides. So what this red sludge means is that your bleach bath is saturated with silver chromate in solution and any stray halide ions in solution are effectively precipitated out before they can get inside your emulsion. Even though some precipitated silver chromate may form in your gelatin layer it comes out easily in the DI bath. After this final DI bath you can then rinse the hologram in tap water to eliminate any dichromate ions if you wish, because there will be no soluble silver ions to cause trouble in a final tap water rinse. (Personally I like having a trace of dichromate in the hologram not washed out because it helps to prevent future printout. However dichromate is quite poisonous and who knows what future use your hologram may be put to particularly with young children around).
Filtration is a good way to make large quanities of water at a known quality. There are many filter types.
Charcoal filtration passes water through activated charcoal. Charcoal is activated by using Oxygen to open millions of tiny pores in the carbon structure. The resulting surface area can exceed 1000 M^2 per gram!
A charcoal filter works by adsorbing impurities it is effective in removing chlorine and carbon based molecules.
- Commonly Available
- Removes Chlorine
- Will not remove Nitrates or salts
Ion Exchange uses salt to take out all of the ions in water and replace them with Na+ and Cl- ions. Also called water softeners. These are not very suitable to rinse water.
- Removes all of the Ca++ and Mg++ ions.
- Inexpensive compared to DI units.
- Removes upto 10 ppm of Fe.
- Must be flushed with water daily.
- The salt must be replaced frequently.
- Adds lots of Na+ and Cl- ions to the water.
- Leaves chlorine intact.
These filters usually use a mixed bed ion exchange unit. It provides very pure water. The cartridges must be recharged by washing the ions from them. Most often they are replaced. Usually a Carbon filter and/or a softener are used ahead of a DI unit.
- Very pure water.
- High flow rates.
- Medium installation costs.
- Expensive to recharge and replace cartridges.
Small RO systems usually consist of 3 or 4 filters. A coarse filter, a fine filter, a charcoal filter and a membrane filter. It is the function of the last filter that gives this system it's name. In order to operate much water is used for providing the osmotic pressure. It is not unheard of to have 2 gallons of waste water for each gallon of RO water.
RO units are slow to produce water so they usually have a storage tank to hold water made in off peak times.
- Filters all ions to around 5ppm or better.
- Filters almost all impurities.
- Wastes lots of water.
- Filter replacement is expensive.
- Requires a storage tank.
Distillation evaporates water and condenses it on a cool surface leaving all of the impurities behind. It provides the purest water in a lab.
- Provides the most pure water.
- Low flow rates.
- High energy costs.