Explaining why film can be exposed after using fixer

<p>I am doing a project and I recently found out that film is allowed to be exposed to light after using fixer. I was wondering how this is possible... and also if anyone has any ideas on what I should do to create something that would explain this process and reasoning?<br>

All I can think of was showing the film but I only fixed and I didnt not complete the rest of the steps. I need help :/</p>

<p>Film records images on crystallized silver compounds. Developer changes the nature of the silver compounds. Stop bath stops the chemical action of the developer. Now (Drum Roll....quote from Wikipedia (Google is your friend)): <strong>Photographic fixer</strong> is a mix of chemicals used in the final step in the <a title="Photographic processing" href="http://en.wikipedia.org/wiki/Photographic_processing">photographic processing</a> of film or paper. The fixer stabilises the image, removing the unexposed <a title="Silver halide" href="http://en.wikipedia.org/wiki/Silver_halide">silver halide</a> remaining on the <a title="Photographic film" href="http://en.wikipedia.org/wiki/Photographic_film">photographic film</a> or <a title="Photographic paper" href="http://en.wikipedia.org/wiki/Photographic_paper">photographic paper</a>, leaving behind the reduced metallic silver that forms the image, making it insensitive to further action by light. Without fixing, the remaining silver halide would quickly darken and cause <a title="Fogging (photography)" href="http://en.wikipedia.org/wiki/Fogging_%28photography%29">fogging</a> of the image.</p>

<p>As Stephen says. The very name <em>fixer</em> implies, that the fixer "fixes" the image.</p>

<p>In the early days of discovery this was the crucial missing step since they could get an image, but it would fade/turn black on exposure to light because whatever was sensitive to light was still there.<br>

By removing the unexposed silver (in the form of halide-- what the fixer does) and leaving the metallic silver (developed) you get a more-or-less permanent image.</p>

<p>It will help if you try to understand what is going on, instead of just following the instructions blindly.</p>

 

<p>As you know, sunlight shining on objects can and will cause a chemical change. Photography is possible because of this action. Years ago it was discovered that three compounds of silver quickly darken with light exposure. These are silver iodide, sliver chloride, and silver bromide. These chemical compounds resemble table salt. We mix them into glue made with gelatin and coat this emulsion onto a clear flexible plastic base. This is film; it is milky white if fabricated in the dark.</p>

<p>The film is placed in a camera with a lens and shutter. When we take picture we allow light to be focused by the lens onto the film. The shutter limits the time the light is allowed to play on the film. Now light consist of particles we call photons. These bombard the silver compound crystals and induce a chemical change.</p>

<p>The developer solution can detect those silver crystals that have been exposed to light. The developer splits exposed silver crystals into their two component parts. The developer ignores silver crystals not exposed to light. Silver metal is liberated from the split (reduced) crystals. The other component is dissolved away by the developer. The liberated metallic silver is opaque and this makes up the developed image. </p>

<p>Now a great many silver crystals remain imbedded in the gelatin emulsion. If the film is removed from the developer without dealing with these unexposed and undeveloped crystals, they will in time self-reduce and darken. This action will destroy the image. To avoid this boo-boo we need to find a chemical that will wash off the undeveloped silver crystals. This is the job of the fixer. The fixer is a solvent for silver salts but not a solvent for metallic silver.</p>

<p>Once the fixer has finished its job, the film no longer contains light sensitive chemicals. The only problem is, the fixer solution is harsh in that it will tarnish the silver image. However if we wash the fixed film with water we can rid the emulsion of this tarnishing agent.</p>

<p>After the film is fixed and washed the image is rendered permanent and not suitable to damage by exposure to light. </p>

<p>How to test and demonstrate the action of the fixer:<br>

As stated, the fixer is a solvent for silver salts. Silver salts are often called silver halides. The halides are a group of elements, and halide is Swedish for salt maker. The fixer is not a solvent for metallic silver in the time allotted for the film to bathe in the fixer solution. Thus, when the film emerges from the fixer the image you see is made up of a clumping of metallic silver particles. Under a microscope they look like tuffs of steel wool.</p>

<p>Now photo paper and photo film are close relatives as both consist of salts of silver suspended in a broth of gelatin. The chief difference is that one is coated onto a plastic film base; the other is coated on paper. As far developing and fixing is concerned, both behave identically.</p>

<p>How to demonstrate for the classroom:<br>

Procure a few sheets of photo paper. You can use film, however, photo paper will lie flat, whereas photo film tends to curl up. Procure a tray of fixer solution. The photo paper or film should be kept in a lightproof box or envelope until you are ready to begin. Again paper is the better choice for this demo.</p>

<p>In ordinary room light, lay out two sheets of paper face up on a table or desk. Quickly place on top of them, objects such as coins or keys. The paper or film will slowly darken due to the action of light; no developer is necessary. The coins and keys shadow the emulsion beneath so the shadowed areas remain unchanged. This darkening happens slowly, so you will not likely notice. After about 5 minutes remove one coin or key. You will see its outline on the paper or film. You are making what is called a shadowgram. If the outline of the object is not bold enough, replace and wait 3 minutes, then inspect again. After a time (likely 10 minute) the emulsion not covered by objects will have reached maximum blackening.</p>

<p>Remove all objects and place one of the sheets in the fixer. The other sheet remains without objects on the desk/table. Swish the sheet in the fixer; this is called agitation. After about 5 to 8 minutes remove it from the fixer and wash it under running water for a few minutes. Blot it off and place it on the table next to the untreated sheet.</p>

<p>The fixed and washed sheet is now fixed (scientific word for rendered permanent). The untreated uncovered paper continues to darken. The areas under the coins and keys, now exposed to light, slowly darken. Soon both areas will blend becoming uniformly dark. The unfixed sheet is now fogged, as all traces of the images are said to have faded. <br>

You have just repeated an experiment performed by Fox Talbot in 1839. He received the idea from John Herschel, a famous astronomer trying to photograph stars. Herschel found that that hyposulfite of soda (hypo) for short would render the images on photo films permanent.</p>

<p>Thanks so much everyone!<br>

Alan this is a great idea and will help me fully to understand the whole process, thanks so much!</p>