In those days, and for many decades thereafter, neither gun crews nor transport were permanent. They had to be hired as they were needed. Master gunners were usually civilian "artists," not professional soldiers, and many of them had cannon built for rental to customers. Artillerists obtained the right to captured metals such as tools and town bells, and this loot would be cast into guns or ransomed for cash. The making of guns and gunpowder, the loading of bombs, and even the serving of cannon were jealously guarded trade secrets. Gunnery was a closed corporation, and the gunner himself a guildsman. The public looked upon him as something of a sorcerer in league with the devil, and a captured artilleryman was apt to be tortured and mutilated. At one time the Pope saw fit to excommunicate all gunners. Also since these specialists kept to themselves and did not drink or plunder, their behavior was ample proof to the good soldier of the old days that artillerists were hardly human. SIXTEENTH CENTURY CANNON After 1470 the art of casting greatly improved in Europe. Lighter cannon began to replace the bombards. Throughout the 1500's improvement was mainly toward lightening the enormous weights of guns and projectiles, as well as finding better ways to move the artillery. Thus, by 1556 Emperor Ferdinand was able to march against the Turks with 57 heavy and 127 light pieces of ordnance. At the beginning of the 1400's cast-iron balls had made an appearance. The greater efficiency of the iron ball, together with an improvement in gunpowder, further encouraged the building of smaller and stronger guns. Before 1500 the siege gun had been the predominant piece. Now forged-iron cannon for field, garrison, and naval service--and later, cast-iron pieces--were steadily developed along with cast-bronze guns, some of which were beautifully ornamented with Renaissance workmanship. The casting of trunnions on the gun made elevation and transportation easier, and the cumbrous beds of the early days gave way to crude artillery carriages with trails and wheels. The French invented the limber and about 1550 took a sizable forward step by standardizing the calibers of their artillery.