Hi Nikolaos,
try some white rice ash. White rice ash does not have a particular aroma, so it doesn´t influence the smell of the burning agarwood (matter of fact, agarwood should not be BURNED but HEATED only!)
Also, you could just burn a lot of coals, and then use that ash to make a small mound into which the glowing coal is set. Then cover the glowing coal with some more ash, so the agarwood is only heated in the hot ash. You will need to experiment a little with this. The chip should not really "burn" but only turn darker, bubbling with resin, and releasing fine wisps of smoke - that is the correct way to enjoy agarwood smoke...
White rice ash is available in good esoteric bookstores, they usually carry incense sticks and incense equipment as well.
Or through amazon:
http://www.amazon.com/SHOYEIDO-60101-Shoyeido-White-Ash/dp/B001B61JXY
I also recommend you use the bamboo coals (for example, "miyakosumi" from Shoyeido).
Bamboo charcoal does not contain any burn enhancers (chemicals such as phosphorus, saltpeter etc.) which will cause the coal to sparkle when ignited. Those chemicals will alter the smell of the coal and thus influence the smell of the wood, too.
Bamboo charcoal will just quietly glow and, step by step, you will see how it glows all over. Once it has turned all white (covered in ash, glowing beneath the ash) it is ready for use.
DO NOT BRUSH OFF the layer of white ash from the coal- it serves as protective layer between glowing coal and wood chip!
Instead, cover with either rice ash; or some ash from earlier burning sessions.
Carefully put the chip on top of the hot ash, and enjoy.
You can refer to KyaraZen´s website (kyarazen.com) - he has lot of wonderful information about the Japanese Incense Ceremony (Mon-Koh).
Or simply check out this one:
http://www.japanese-incense.com/contents.htm
Kind regards
Thomas S.