Roasting tea serves a few purpose, here are the ones I know of:
1, small commercial operation to change flavor, prepare for storage and/or aging.
2, home roasting using rice cooker to reduce moisture for storage and/or aging
3, roasting before brewing for flavor enhancement
I speak of what I know, and what I know is not the only way, nor it is precise. However it's a learning process, it serves the purpose of what I try to achieve at this point. Of course my expectation of end result will raise as experience accumulates and that my skill improves along time.
There isn't an exact way or formula to roast tea. The basics are temperature and time control.
Small commercial operation:
My way for initial roasting is using a semi commercial electric tea roaster, 80 C for 1 hr to warm up, depending on the type of tea (maturity and shape of the leaves, original roast and fermentation) and the final taste you want, turn up temp for actual roasting for any where between 30 min to 1 hr, temp can be anywhere between 90 to 110 c, then turn down temp to 70 to 80 for 5 to 8 hrs. It's now time to taste. I usually put aside some of it at this point if I am not satisfied with the result and proceed to another session of roasting. It's trial and error from this point on. I have burn some batches, under roasted some, and some came out quite nice but I still wonder if it could get even better. The key is as long as you did not mess up (burn) a batch, you can manage to make it better.
Highest temp for re-roasting tea is not to exceed 110 c, unless you like charcoal flavor tea. Patience is virtue my tea friends. Bear in mind the bottom line is to decrease moisture for better aging, flavor is second to it. For fisted oolong, longer it takes at lower temp, because you do not want to burn the exterior while the interior still contain moisture.
Green tea in bamboo container above. Breathing hole in the middle for air circulation. I just started this batch today, will be stored away in sealed container for the next 10 yrs. If you remember this in 10 years, ask me for some sample of aged Mao Jian. :P
Roaster does not come with cover, I use layers of paper to cover opening. I will look for a bamboo cover soon.
Above picture shows before and after roasting. Left is before, right is after. This is TGY of 07 which Will acquired from China in Oct. One of the medium grade TGY with typical green grassy flavor before roasting. Roasted TGY is slightly more brown than the original, but the difference is rather small. However the taste is much different. Roasted TGY lost most of the grassy taste, sweet roasted rice aroma is the new transformed flavor, much more palatable than the original flavor. The whole duration of roasting went on for about a week, with initial roasting (low, high then low temp) about 7 hours, another 4 hours (low) the next day, rested for 2 days, 1 hr (low) every other day for the rest of the week.
Home roasting using rice cooker:
Rice cooker must have "keep warm" function. You can roast 1 to 2 lbs at a time depending on the shape of leaves. Loose strands take up more space, hence less tea can fit in rice cooker pot. "keep warm" for 8 hours if you just want to loose excess moisture. To change flavor, "keep warm" for 1 hour, "cook" for 30 min to 2 hours depends on original roast, shape of leaves and how you like the resulting flavor, then "keep warm" for 6 hrs. Test result with tasting, repeat if desired more roasting. Do not cover with lid when roasting, cover entire pot with a few layers of clean paper free of odor. Moisture can escape through paper is the purpose. For greener teas, leave center open by creating a hole in the middle of the pile.
If you try to change the flavor of tea by using high heat, please do beware the flavor will continue to transform for months after roasting.
Roasting before brewing:
Here is an article I wrote a while back using a Yixing roaster.
Happy roasting guys!