In an interview he explains this choice:
DA: How did you first get into the use of liquids such as tea in your work?
CG: “I have drawn with fountain pen for many years, often with plain water washes. When I decided to leave my post of creative director at an embroidery firm to pursue a love of drawing I experimented with liquids such as brandy.
“I liked the effect this had on the inks I was using but decided that an alternative that wasn’t such a wasteful crime would be a better option so I started experimenting with different types of tea.”
DA: What does using such materials enable you to do that conventional painting techniques alone do not?
CG: “Colour is a difficult area for me and something that I have become more confident with in recent works. My confidence in using different materials is growing but the benefit of working with a medium such as tea is that it creates a naturally harmonious palette.
“I can introduce vibrant colours but the tea does the job of mixing with these to create a balance in the work. I know this could have been achieved with a reduced palette of colours but there is something about the whole process of brewing the tea and working with a colour that becomes more intense or darker in tone as the work progresses.” (keep reading the interview here)
Interesting that he is an English mans. It would not be more difficult to keep on using this technique if good and diverse qualities of tea were difficult to find.