What is the black silt in bottom of my film developer?

Question

I'm new to developing sheet film and I've just noticed that a fine black silt is accumulating in the bottom of my developing tray. My only guess is that it may be residue from the antihalation layer. I'm reusing the developer for several sheets, but within recommended usage and it seems to be working just fine. The silt stays in the very bottom and rinses out easily after I pour the developer back into its bottle.

This is HP5+ and Delta 100 in Ilford's ID-11.

Update: I decided to experiment a little and put some developer and water on separate areas of the back of a sheet of film. The water loosens the antihalation coating which rubs off on a rag, showing a black residue. The developer completely dissolves/transforms the antihalation layer, leaving no visible residue when I wipe off the excess. From this, I assume that the silt is not from the antihalation coating, but from some other reaction.

From the informative responses from Alan Marcus, David Berry and Jindra Lacko, my current guess is that it is some other precipitate, possibly from the tap water I'm using. Though I think my tap water is pretty good as such and I've never had a noticeable problem in the past, I think I'll go with distilled from now on.

Answer 1

The developer solution is somewhat complex as to the reactions occurring. All commercially available formulas add proprietary goodies to combat hard water. Often, dissolved minerals from the water will precipitate out of solution. All water contains dissolved oxygen (the developer is mainly water). The developing agent has an affinity for oxygen and this reaction destroys the developer. The developer is a derivative of benzene which was initially extracted from coal (now likely synthesized). As the developer reacts with oxygen, the developer reverts to coal tar.

All formulations contain a preservative which is sodium sulfite. The preservative reduces aerial oxidation and reacts with oxidized developer to render it non-staining. After a time, the preservative is exhausted and gives up. We replenish developers with a modified formula that rejuvenates. Other solids will precipitate out of solution especially when the developer is near exhaustion.

Most films sport an anti-halation coat made of dye. Some motion picture films are back coated with a Rem-Jet (removable jet black) make of lamp black. The Rem-Jet is held in an acid plastic that releases in an alkaline solution. It is not likely that the sheet film you are developing has a Rem-Jet.

Answer 2

2 possible reasons, explained in following extracts :

Focal Encyclopedia of Photography : Digital Imaging, Theory and Applications, History, and Science. p.707, 4th edition © 2007 Elsevier Inc. by Michael R. Peres Editor-in-Chief Page displayed by permission of Taylor & Francis. Copyright. 

*[By the way, Hydroquinone is ingredient of ID-11 developer.] ____________________________________________

" Abstract: Calcium salts may be introduced into photographic processing solutions from (a) the water supply, (b) the emulsion, or both, and these salts combine with some of the developer constituents to form insoluble compounds which may appear as (1) a sludge suspended in the developer or accumulated on the filters, (2) a scum on the film, or (3) a scale on rollers, sprockets, racks, and the walls of tanks. The control of the water supply will reduce the quantity of these precipitates and the scum may be removed by suitable acid rinse or acid fixing baths, but calcium-sequestering agents are often used for more complete control. When selecting these agents, their calcium-sequestering power, stability, photographic effect, and their effect when carried over into the fixing bath must be considered, and on the basis of these requirements, sodium tetraphosphate was found the most suitable.† — Appropriate quantities of sodium tetraphosphate added to the developer were found to (1) prevent sludge formation in mixing, storing, or use of the developer, (2) prevent the formation of scum on the film when in the developer, and (3) greatly diminish the rate at which incrustations accumulate on the tank walls, sprockets, and mechanical parts. — If developers are stored at high temperatures, . . . "

http://ieeexplore.ieee.org/document/7252099/?reload=true

Published in: Journal of the Society of Motion Picture Engineers ( Volume: 43, Issue: 6, Dec. 1944 ) Page(s): 426 - 441 Date of Publication: Dec. 1944 Print ISSN: 0097-5834 DOI: 10.5594/J07990 Publisher: SMPTE

R. W. Henn ; J. I. Crabtree

Answer 3

If you are concerned about deposits from the antihalation layer in your developer you can presoak your film for 2 to 5 minutes (and have the antihalation layer wash there). The benefits of presoaking are somewhat contested, but you can give it a try.

I personally would not lose sleep over slightly muddy developer. Good old Rodinal is famous for turning dark and forming precipitate while still working perfectly. ID-11 is of course different, but as long as you are within recommended usage you should be OK.