In Control panel you'll find the Printers and Faxes icon. Click on that. On
the Epson R320 icon right click. Click left on Properties at the bottom of
the menu.
Although I don't have R320 software in front of me I use a lot of Epson
models. On the software settings see if there is an 'advanced' setting page
and look to see if there is a 'print in black' box that might be checked.
You might also have the option of printing in 'Economy' mode, which would
save you on ink usage, though printing is lighter and not as fine quality.
