Thank you guys for all your suggestions, they helped me solve the problem
I succeeded in recreating the error. Printing 5 A4 B/W test photos didn't make the magenta cartridge fail, but a magenta purge page created in MS Word with a large rectangle with magenta fill color, RGB values 255-0-255 did the trick. The first page printed OK, the second produced this result, the first picture is reduced in size, the second cropped, click to enlarge:
and a cropped image:
After the cartridge had failed I examined it and found that the outlet sponge and the encircled area were a lighter color than normal. Here is a scan of the failed cartridge and a normal one, click to enlarge :
The failed cartridge still held about 4mm or 5/32" of ink in the reservoir. I left it in a cartridge clip overnight, and checked it in the morning. It had not changed in appearance. I put it back in the cartridge clip for protection and tapped it hard against my desk, both with the ink outlet and the chip facing down. Suddenly the ink in the reservoir disappeared into the sponge and the ink outlet and the encircled area turned into the normal darker color within a minute or less.
I must say
stratman was perfectly right about the ink starvation and that my previous tests had not stressed the cartridge enough. Also
ThrillaMozilla's suggestion to tap the cartridge worked perfectly. I don't think the vent was clogged. Holding the cartridge over the kitchen sink, after having tapped it, and blowing into the vent caused ink to exit from the outlet in a normal manner. I also think the sponges were in no need for reconditioning.
I think the cause of the problem was either that the sponges had been shifted in position by my poking with a needle or that I had failed to maintain pressure on the SquEasyFill bottle when withdrawing the needle, causing an air lock.
I'm glad the problem was a cartridge and not a clog in one of the tiny ink passages in the upper black part of the print head.
Re hard water and conditioning:
I also live in a hard water area, so I use demineralized water for flushing cartridges. When you use a Freedom Refill adapter, you can flush a dye ink cartridge completely with less than 200 ml of water. When bought in 5 liter quantities demineralized water is not very expensive, and I need not worry about lime deposits inside the cartridge. But I think you can flush with hard water till the cartridge is clean and finally flush with a little distilled/demineralized/deionised water. I think you will only get lime deposits inside the cartridge, if you leave the hard water in the cartridge and allows it to evaporate. Demineralized water will not dissolve lime deposits already in the cartridge, some acid like acetic acid will be needed. But maybe hard water is not that big problem, I have flushed cartridges with hard water without subsequent flushing with demineralized water without ill effects.
I don't know very much about contitioning of sponges. That is something I need to learn more about. But I think sponges should not be completely dried after flushing. If too dry, they may be hard to wet with ink. Compare a completely dry rag with a slightly moist rag. The dry rag is hard to wet, the moist rag absorbs water readily.
Re German method vs Topfill method:
Even if I have refilled 50+ cartridges with no problems using the German method, this incident has made me consider changing to the the Topfill method. With the topfill method you don't risk dislocating the sponges, although I use sharp needles to reduce that risk. My primary reason for preferring the German method is that you need not tamper with Canons sealing of the cartridge if you purge using a Freedom Refill adapter. But I think a low profile silicone plug
plus aluminum tape make a secure seal. I had a CLI-521 C cartridge that had been purged after removing the plastic ball and sealed with a plug plus aluminum tape. I topfilled that cartridge pr
miklings instructions and using a SquEasyFill bottle. It was actually easier than German refill and the weight after refill was also OK: 20.4 grammes. One more bonus: With topfill you can use blunt needles without problems. So I wonder if I should change my preferred refill method.
