I received this good info from Canon support on 01/09/2013 re PIXMA MX882:
The bigger black ink tank contains pigment based black ink. This ink provides dark and sharp black text on plain paper. The bigger black ink tank is used when the paper type is set to plain paper.
The smaller black ink tank is a dye based ink. This ink provides greater contrast when printing photographs.
The printer heats up the printhead to put ink on the paper. It uses ink to keep the printhead cool. If there is no ink in the ink tank, the printhead would overheat and become damaged.
This is why the printer will not print until an empty ink tank is replaced.
Each ink tank has its own purpose. You cannot switch to one when the other is empty. The cartridge would need to be replaced.
The PIXMA MX882 will use color ink tanks to print colored text. Even if the grayscale option is checked in the printer driver, if the original text is in a color other than black, the PIXMA MX882 will combine the cyan, magenta and yellow ink tanks together to make a composite black. This is why your color ink cartridges are still being used. Duplex printing also uses a combination of colors to create black, this is due to ink drying time between pages when printing in duplex mode.
To use only the Pigment Black or large black ink tank, the following settings must be used:
1. The original text must be black.
2. The paper setting must be set to Plain Paper.
3. The Duplex printing option must be disabled.
4. The Grayscale printing feature must be disabled.
