Okay, let's say you are merlin or something and zapfry the block of lead into pure energy. That energy would be in the form of Electromagnetic radiation as Doc Said. Keep in mind though, that electromagnetic radiation is affected by gravity too. Now, on the small (read: trivial) scales of everyday light, radio, and gamma radiations, this effect is miniscule. But, the energies you're talking about (900,000,000,000,000,000 Joules, or ~500 megatons) would be affected by gravity significantly.
The effect of gravity on electromagnetic radiation however, isn't attraction, but it tends to bend the paths of radiation waves. So, your energy beam would tend to curve and more importantly, spread out. Now, your energy loss would come from the energy required to counteract the spreading effect of gravity and keep your beam headed to one place. This would come out to being equal (in a fairy-tale lossless world) to the energy required to lift the matter against gravity. So, basically, you wouldn't loose mass, you'd just have to expend energy anyway.
