Intended for vertical cracks, Aluminium Heads (or Aluminum Heads if you're a Yank) are designed to be quick to bash in and so are particularly useful in awkward situations or when you are really tired, but are best suited to good placements as their holding power is not as good as Copper Heads.
Approximate head measurements:
Size 1: 2cm x 1cm, 10g
Size 2: 2cm x 1.2cm, 26g
Size 3: 2cm x 1.4cm, 28g
Size 4: 3cm x 1.7cm, 34g