Why is there a second black ink tank? -

What does the second black ink tank do? I m talking about a Canon (MP540), and there is a big black tank, and a small one. The big one runs low really fast, but the small one appears unused, even less than the color tanks. I assumed the small one is for photographs and the big one for text, but then why do my color tanks go low faster than the small black one. And if the big black one runs out, will the small black one take over? Any ideas?

One is pigment black and the other is dye based black. They both have different functions and pros and cons. I have two blacks in my MP610. The big one is the pigment one and the small one is dye based. And no, the big one will not take over if the smaller one runs out as it is programmed to know which place to take which colour it wants and as such it does not take the next colour available to it it will just not print properly. You are correct that one black is for text and the other is used is designs. The pigment ink is more water resistant than the dye based one which is water based and as such runs when wet. This has the two sorts of inks http://www.oddparts.com/ink/faq19.htm

If the printer is set to grayscale, it uses the big one, if it′s set to colour it uses the small one. Unlike other printers which do not have an extra black cartridge for colour printing, you will get real black if you have big areas of black on a colour print out. Printers with only two cartridges i.e. mix black out of three colours and it looks mostly brownish.