Purpose of Agitation

<p>Hello all,<br>

I am new to photography and am a student taking an Intro to Photographic Medium class. We have been working on projects over the past few weeks and have been learning how to process our film and make prints. I was curious about why we have to agitate the film during processing. Why are the times so specific for each chemical being used in the process? Any feedback would be greatly appreciated.</p>

<p>agitation: When film gets developed, the developer solution directly in contact with the film becomes exhausted. The agitation brings fresh solution in contact with your film.</p>

<p>developing: The times are specific because the amount of developing changes with film / developer / temperature combinations. BW films can be all over the place, as every kind of film is different and there are many choices for developer. With C-41 (colour) and E-6 (slide) all the film was engineered to be the same to allow a constant process. BW, E-6 and C-41 film development time changes with developer temperature because it becomes higher in activity with an increase in temperature.</p>

<p>Clear anything up, or did I make it worse? :)</p>

<p>Oh, welcome to film!</p>

 

<p>Additionally, agitation can prevent air bubbles from being trapped between the developer and the surface of the film during the development process, preventing the development from occurring. Development is a series of chemical reactions....each chemical has specific properties and each chemical reaction, therefore, may be different - depending on things like concentration of the chemicals, pH of the solution, temperature and agitation, to name a few. Think of it as making a cake...different ingredients will change the baking times if the temperature remains constant.</p>

<p>But at times with lesser agitation with some developers mostely hyper diluted we get what is known as Stand development. It it allows the highlights to rest as shadows develope Semi Stand is most ofen used but Bromide dragging happens with some films and developers. So to confuse you more. Lesser agitation for shadows more for highlights.</p>

<p>C41 and E6 have specific times, but they may not be the optimal time for all films.<br>

Standardizing the time is convenient, but it is a trade off. There are C41 and E6 films designed to be pushed.<br>

I have never seen it on my film, but it is supposed to be that too little agitation gives streaks as the exhausted developer, heavier than water, falls down the emulsion surface slowing development. <br>

But I most often use Diafine for film, which is less sensitive to under agitation. (But you do need to keep air bubbles away.)</p>

<p>I believe we opened a can of worms. </p><div></div>

<p>The light sensitive goodies are salts of silver. There are three. These are crystals, listed in order of sensitivity, silver bromine, silver chlorine, and silver iodine. Photo films are chiefly silver bromine. Different ratios yield different sensitivities. These are all members of the halogen family of elements. Halogen is Swedish for salt maker. These salts resemble table salt, however the crystals are much smaller and all have low solubility. They are coated onto film or paper using a glue binder comprised of gelatin. Gelatin is chosen because it has low solubility, high transparency, and it is flexible. Gelatin, under the microscope resembles spaghetti; it is a long chain polymer. The light sensitive crystals are imbedded in the spaghetti strands. When wet, gelatin swells like a dry sponge plunged into water. This action opens up the structure allowing the fluids of the process to enter and percolate about. It replaces egg whites that were formally used as the binder.<br>

<br>

The silver halogen compound is held together by a chemical bond. Exposure to light will weaken this bond. The developer solution is able to identify crystals with weakened bonds and reduce them to their two component parts. Metallic silver is left behind and the halogen component is dissolved away into the waters of the developer. It is the metallic silver that makes up the black & white image. The developer, being selective, does not immediately reduce unexposed silver halides, however, given sufficient time, it will reduce all. Thus the developer step is timed; otherwise the entire film will be blackened (fogged).<br>

<br>

As the developer works on the exposed silver salts, exposed crystals are reduced. The halogen component is chemically a restraining agent. In other words as reduction of crystals continues, more and more retainer is liberated. We agitate to cause fresh developer solution to enter the gelatin structure. This fluid movement flushes out the restrainer and forces fresh developer to enter and do its job. <br>

<br>

Temperature and agitation play key roles. Higher temperatures accelerate chemical reactions and cause the gelatin to swell at an elevated pace. The bottom line is, the entire developing process is based on infusion of fluids and the time it takes for the chemicals to do their task. </p>

<p>Thank you all for your responses. I do understand agitation and the developing process more now. I now realize how much chemistry really goes into this type of photography.</p>

<p>There is also constant agitation like the JOBO systems. In these cases times are reduced and I only use my machine for color films.</p>

Jobo rotary processors can be used for B&W film development in a higher dilution or for developers designed for 24C

temperature development, like CG-512/RLS 1+4 or Microdol-X 1+2. High acutance developers are less suitable for this

type of agitation.