1.  Field of the Invention
The present invention relates to a liquid jetting apparatus which jets liquid from nozzles and a maintenance method of the liquid jetting apparatus. 
2.  Description of the Related Art
As described in Japanese Examined Patent Publication (Kokoku) No.  60-60054, in a color image forming apparatus which forms an image by using coloring materials of a plurality of colors, it has been known that, when shortage of any one color material of the plural color materials occurs, the image formation with the color material is prohibited and the image formation with the other color materials is enabled. 
From the same viewpoint described in Japanese Examined Patent Publication (Kokoku) No.  60-60054, in a liquid jetting apparatus, for example, in an inkjet printer, when shortage of color ink occurs, printing with the color ink is prohibited and only printing with black ink is enabled until the color ink is refilled, which offers a user improved convenience. 
Here, in the case where printing with one ink (first liquid) is prohibited and only printing with the other ink (second liquid) is enabled as described above, recovery operations such as flashing in which the second liquid is jetted by driving a liquid jetting head and purging in which the second liquid is sucked out from nozzles to be discharged to the outside are performed in the liquid jetting head, in order to prevent an increase in liquid viscosity in the nozzles from which the second liquid is jetted.  At this time, if the flashing and purging are also performed with respect to the nozzles which jet the first liquid, a great amount of air is led from an empty ink supply source into a liquid jetting head which jets the first liquid, which makes it difficult to recover the liquid jetting head of the first liquid into a state where it can jet the liquid. 
On the other hand, if the flashing and purging with respect to the liquid jetting head which jets the ink as the first liquid is totally prohibited, and if a user often uses the second liquid, the liquid jetting head of the first liquid is left unused for a long period, and the liquid jetting head of the first liquid is exposed to a dry state while the liquid jetting head of the second liquid is operating.  Accordingly, the increase in the liquid viscosity in the liquid jetting head of the first liquid progresses with time.  Therefore, depending on the degree of the increase in the liquid viscosity in the jetting head, it is necessary to perform the flashing or the purging when the first liquid is refilled, for the purpose of solving the problem of the increase in the liquid viscosity in the liquid jetting head of the first liquid.  However, if the flashing or the purging is indiscriminatingly performed at this time, depending on the degree of the viscosity increase of the ink liquid in the liquid jetting head, there is a risk that the liquid may be discharged wastefully or more than necessary, or the increase in the liquid viscosity may not be fully solved.