TECHNOLOGY FOCUS - POLYMERS - Preferred composition: the microspheres comprise a polymer shell of copolymer of acrylonitrile, vinylidene dichloride, and methyl methacrylate, encapsulating butane as a blowing agent. The material comprises an explosive material, e.g. ammonium nitrate (preferably as a highly concentrated or aqueous solution. The property changed comprised developing porosity in the material. The microspheres are mixed with the ammonium nitrate when forming prills from it. The blowing agent is released while the prills are in an early stage of hardening. The mixture of material and microspheres are treated at an elevated temperature causing the microspheres to over expand, allowing the blowing agent to escape, where less than 5 wt.% blowing agent is retained. Microspheres encapsulate 11-80 wt.% of blowing agent. The blowing agent comprises a mixture of gases, some of which inflate the microspheres, others escaping from them. EXAMPLE - A porous prilled ammonium nitrate product was prepared without microspheres, using commercially-available microspheres, and using specially prepared microspheres (Expancel (RTM)) containing additional isobutene as blowing agent. The apparatus used comprised a tower using a shower-type prilling nozzle. Dry prills were analyzed for each product, with analyses of the microspheres included. The amount of isobutene present in commercial microspheres was 11%, whilst that in Expancel was 31%. The loose bulk density of the prilling tower bottom product was 0.82 g. per ml. where no microspheres were present, 0.63 g. per ml. where commercial microspheres were present, and 0.62 g. per ml. for Expancel, where the gravimetric microsphere content was 0.30% for commercial microspheres and 0.08% for Expancel. The loose bulk density of dry prills was 0.72 g. per ml. where no microspheres were present, 0.59 g. per ml. where commercial microspheres were present, and 0.61 g. per ml. where Expancel was present, with an oil absorption of 5.9% where no microspheres were present, 6.5% using commercial microspheres, and 10.0% for Expancel.