Different types of bitumen have been used: blown bitumen (partly oxidized with air oxygen at high temperature after distillation, and harder) and direct distillation bitumen (softer).
dˈɪfɹənt tˈaɪps ʌv bɪtˈuːmɛn hɐvbɪn jˈuːzd: blˈoʊn bɪtˈuːmɛn pˈɑːɹtli ˈɑːksɪdˌaɪzd wɪð ˈɛɹ ˈɑːksɪdʒən æt hˈaɪ tˈɛmpɹɪtʃɚɹ ˈæftɚ dɪstɪlˈeɪʃən, ænd hˈɑːɹdɚ ænd dᵻɹˈɛkt dɪstɪlˈeɪʃən bɪtˈuːmɛn sˈɔftɚ.