A short dictionary of emulsion making and coating terms
Anode, Anion – This is the negative or ground pole of a battery, the black lead on a Volt Ohm Meter (VOM), or a negatively charged ion. Bromide is an Anion
Baryta - Barium Sulfate paste dispersed in gelatin with a humectant and hardener. It is used for a subbing on paper support to give a smooth surface with high reflectivity. Some types of baryta contain ingredients that fog emulsions and must be prepped before coating. Bad Baryta FB paper is characterized by flaking off of the Baryta layer and coated emulsion during processing or afterwards. Sometimes it cracks. This indicates low levels of humectants in most cases, or improper drying conditions at some point. Barium ion is a strong poison, but as the Sulfate salt is so insoluable it is rendered harmless.
Bone Gelatin - Ossein gelatin is derived from cattle bones by lime treatment. It has an isoelectric point of about 4.5. Most gelatin in use today is of this type. Minimum swell is at the isoelectric point.
Bridge – This is a salt filled tube placed between the emulsion making kettle and the reference electrode to prevent leakage and contamination of the kettle with the salt released by the reference electrode which is normally a double junction Ag/AgCl electrode. The salt in the bridge must be inert. Usually KNO3 (Potassium Nitrate) is used. The bridge is plugged with a porous material to minimize leakage.
Cathode, Cation – This is the positive or red lead of a VOM, or a positively charged ion. Silver is a Cation.
Chemical Sensitization – This is any process by which chemicals are added to an emulsion to improve the inherent sensitivity of the emulsion with no change to its spectral characteristics.
Coagulum - This is the mass that forms when you have made an emulsion using PA gelatin and you have added sulfuric or nitric acid to the cool emulsion. It can be redispersed by the addition of sodium or potassium hydroxide (NaOH or KOH).
Constant Volume (CV) - A method of using a UF wash device to allow washing to take place during precipitation. By this means, the volume of the emulsion is kept constant and the vAg may also be kept constant. It can also be done after precipitation during a separate wash step where salts are removed with water, but the volume is kept constant by addition of Distilled Water.
Digestion - A process by which an emulsion is heated in the presence of a silver halide solvent such that fine crystals are dissolved and precipitate on larger crystals. This is usually Ammonia or ammonium salts.
Dispersion - CHEMISTRY (A suspension of a solid in a liquid) PHOTOGRAPHY (A suspension of one liquid in another such as fine oil droplets in water)
Dopants - These are materials that change the photographic properties of an emulsion. They change curve shape, contrast, reciprocity failure, latent image keeping (HIRF or LIRF), and raw stock keeping.
Doctors - These are materials hat change the physical properties of the emulsion melt that is to be coated. They include surfactants and hardeners which change viscosity and spreadability. Generally, Dopants added with Doctors just before coating may be called either or simply all of these may be referred to as Addenda. Doctors are referred to in some literature as Finals.
Emulsion - CHEMISTRY (A suspension of one liquid in another as oil droplets in water) PHOTOGRAPHY (A suspension of a solid in a liquid)
Finishing - See Chemical sensitization. However, this particular form of Sensitization is only carried out after the emulsion is washed. This was a unique Kodak invention disclosed in the 40s or 50s much to the amazement of the general photographic science community. There are few publications or patents on this subject, and it is very complex, almost as complex as making an emulsion itself.
Hardener - This is a special chemical added to an emulsion to promote cross linking of the gelatin and raise the melting point. Gelatin melts at about 68 degrees F (20 degrees C). Typical hardeners include chrome alum, glyoxal, formaldehyde, mucochloric acid, glutaraldehyde and succinaldehyde. Hardening takes place during drying of the coating, but can take place during long term keeping (afterhardening). Hardening is slowed by acidic pH and is sped by alkaline pH. More modern hardeners are in use by the major manufacturers.
Isoelectric point – This is the point at which gelatin has the minimum swell, best hardness and is balanced internally for charges on the acid and base groups. It is approximately pH 4.5 for bone gelatin and about 9.5 for pig gelatin.
ISO Wash - A method of washing an emulsion using either strong salt solution or acid or salt with acid combined with Phthalated (PA) gelatin. It forms a coagulum of emulsion and gelatin which precipitates and may be redispersed by adding alkali to balance the charge on the gelatin. While coagulated, the emulsion can be easily washed by adding distilled water.
Latent Image Keeping (LIK) - This is the stability of the image to change after exposure but before processing. The best is to have no change whatsoever. It is often referred to as LIK.
Make Up Gelatin - This is gelatin added to the emulsion prior to coating to adjust the melt to the proper viscosity and gel content.
Melt - This is the emulsion with all addenda added and ready to coat. It is molten and is usually at 100 deg F or about 40 deg C.
Noodle Wash - A method by which an emulsion is cut or sieved into small noodles or cubes and then washed with distilled water until all excess salts are removed. This step is usually followed by readjustment of the salt content for proper keeping, and then the emulsion is finished.
PA Gelatin - A type of modified gelatin that has been 'phthalated' with an organic acid called phthalic acid. It makes the gelatin sensitive to acid and base (to pH changes), allowing the gelatin to be coagulated by addition of acid and redissolved by addition of base.
Pig Gelatin - Gelatin derived from pig skins and finished with an acid treatment. These gelatins have an isoelectric point of about 8 - 9.
pAg - Negative log of the silver ion concentration. This is often reported as an indicator of the conditions in the precipitation or for final conditions. It is not very accurate.
pX - Negative log of the halide ion concentration (Cl/Br/I). This is rarely used as it is not indicative of a 'real' condition in making emulsions.
RC (Resin Coated) paper - This is a paper that has been coated on both sides with an extruded plastic polymer to inhibit chemical uptake into the paper fibers. More recent papers are using more resin and less paper. It also has ingredients to inhibit oxidation and degradation from light. This resin will not accept a gelatin coating directly. It must be either bombarded with a high electrical charge before coating, or must be bombarded and coated with a gel subbing before use.
Reciprocity failure - this is a change in either speed or contrast of a photographic material as a function of exposure time. There are two types HIRF and LIRF related to High Intensity and Low Intensity light. This and LIK (Latent Image Keeping) are often controlled by the addition of metal dopants to an emulsion during the precipitation.
Ripening - A process similar to digestion but carried out by heat treatment alone with only excess silver halide present to assist the reaction.
Spectral Sensitization - This is the addition of special organic chemicals to an emulsion to change the sensitivity to a different wavelength of light. This includes Ortho- and Pan- sensitivity among others. The chemicals are custom synthesized and are very very expensive.
Surfactants - High molecular weight organic materials that may be uncharged or charged (plus or minus) and which are similar to soaps. They control viscosity and improve spreadability.
Titanox - Titanium Dioxide (rutile or anatase) is the reflective subbing used in RC papers. It has a high UV absorption which makes it ideal to protect images from UV, but on the other hand it makes the energy transfer to the resin easier leading to decomposition of the resin. It must be mixed with antioxidants (usually free radical chain stoppers) to prevent premature cracking and crazing of the resin.
UF Wash - Ultra Filtration. This is a method by which reverse osmosis is used to remove salts from an emulsion either while it is being made or afterwards, and is used in place of either the noodle wash or the PA wash. It is used exclusively at Kodak now.
vAg - This is effectively an offshoot of pAg with much more sensitivity. It is a measure of the conductivity of the emulsion in millivolts and supplies up to 5 digits of accuracy. It is used exclusively at EK, but, as far as I know, it is not widely used elsewhere and is often misunderstood due to the lack of publications on the subject.
VOM – Volt Ohm Meter used to measure voltage or resistance in a circuit. A pH meter is a fancy tricked up VOM.
Web - This is what the moving film or paper is called as it is being coated and through the finishing operation until slit into individual rolls. An entire roll is called a master roll.