Canon MX920 - disable PK black cartridge

The PGBK black cartridge is much bigger than the PK black. I bought the bigger one because I print a lot of black & white. However, I now find out that it will only use the PGBK for printing photos. Is it a different kind of ink, or can I get my computer to default to the PGBK for everything I do? What would be the downside of that, if any?

The PGBK is described as "Canon Pixma Pgi-550Pgbk Ij Cart Phto Blk. There are differences between ordinary black and photo black:

• Photo black is usually a dark gray, to provide gradation between black and the white paper.
• Photo black may be more glossy, for use with glossier colors when printing photos.

That said, it might be possible to force use of photo black in printing text by changing Printer Preferences. The dialog below shows that for an Epson printer under Windows OS as an example; you can also make changes in the Color Management dialog or in the CUPS Printer Settings dialog under Linux. Doing so has the disadvantages that text looks a bit lighter and that printing is slower.

BTW, If ink economy is an object, you might try compatible ink cartridges, or ink refill kits, though some require resetting the cartridge type or low-ink warning.

The MX920 takes PGI-250 or PGI-255 PGBK (pigment black cartridges) and CLI-251 BK (photo black cartridges), which are available in standard and extra large sizes (various XL designations). See the Canon manual here.

Pigment Black vs. Dye Black

The photo black is a dye-based ink, which means it is a colored liquid. Pigment black is a pigment-based ink, which means it has particles of pigment suspended in the liquid. Pigment ink has to be sprayed through a bigger nozzle, so the droplets are bigger. The pigment particles also leave a much denser color on the paper.

The smaller droplets and less-dense color of dye-based black make it more suitable for photo printing because it is much easier to get fine gradations of black. The larger droplets and denser color of pigment-based black make it better for printing text. Text characters are large compared to photo detail and you want them as black as possible (photos typically contain very little actual black).

Photo black and even the dye-based colors are used to fill in the rough edges of the pigment-based characters. So you use a small amount of photo ink when printing text.

Pigment black isn't ordinarily used for photos. The droplets are too big to get the detail control needed. And if you are printing on photo paper and want to use pigment black for better fill and darkness in the darkest portions of photos, the surface appearance is different. In fact, not much photo black is used in photos; it supplements black created from the other colors to make it darker and to correct off-hues.

Your Problem

So back to your question, the pigment black cartridges are usually sold in a larger size because general-purpose printers typically print a lot more text than photos (you can buy a photo printer if you are mainly printing photos, and they do a better job), and ink is applied heavier in text printing, so you go through it faster.

You say, "I now find out that it will only use the PGBK for printing photos". That shouldn't be the case. None should be used for photos. If Canon figured out a way to use some supplementary pigment black on photos, it would be a tiny amount compared to the dye-based inks, and it has a dye-based black for that purpose.

It's been a long time since I used Canon printers, but typically, there is some form of mechanical key that prevents sticking cartridges in the wrong place. But if you were able to stick a pigment black cartridge in a photo black slot, the ink would likely clog the nozzle.

Assuming all of the cartridges are in the right slots, it isn't clear what you're seeing. The printer would likely be malfunctioning if it is using pigment black on photos rather than photo black. And it should use photo black for text only in tiny amounts, particularly around the character outline, unless you're printing an image of text.

Is it possible that you are misinterpreting what you're seeing? If you can add more detail to the question to describe the printer's behavior more explicitly, and what your seeing that leads you to conclude that pigment black is being used only for photos, maybe we can shed more light on what's going on and how to fix it.