pure silica thread in a 1-inch grid, then coated with a high-purity silica coating. The blankets were semi-rigid and could be made as large as 30" by 30". Each blanket replaced as many as 25 tiles and was bonded directly to the orbiter. The direct application of the blankets to the orbiter resulted in weight reduction, improved durability, reduced fabrication, and installation cost, and reduced installation schedule time. All of this work was performed during Columbia's first retrofitting and the post-Challenger stand-down. Though the orbiter's thermal protection system and other enhancements had been refined, Columbia would never weigh as little unloaded as the other orbiters in the fleet. The next-oldest shuttle, Challenger, was also relatively heavy, although lighter than Columbia. Markings and insignia {{multiple image | align = right | direction = vertical | image1 = B747 with Space Shuttle on it from above.jpg | width1 = 250 | image2 = Endeavour after STS-126 on SCA over Mojave from above.jpg | width2 = 250 | caption2 = A top view of Columbia and Endeavour is shown. Note that Columbia'''s chines are black unlike the white ones of Endeavour and the other orbiters. }}Columbia was the only operational orbiter with black chines. These were added because at first, shuttle designers did not know how reentry heating would affect the craft's upper wing surfaces. The chines allowed Columbia to be easily recognized at a distance, unlike the subsequent orbiters. The chines were added after Columbia arrived at KSC in 1979. The only other orbiter with black chines was Pathfinder, but it was a cosmetic test article and only gained it when was refurbished. Additionally, until its last refit, Columbia was the only operational orbiter with wing markings consisting of an American flag on the port (left) wing and the letters "USA" on the starboard (right) wing. Challenger, Discovery, Atlantis, and Endeavour all, until 1998, bore markings consisting of the letters "USA" above an American flag on the left-wing, and the pre-1998 NASA "worm" logotype afore the respective orbiter's name on the right-wing. Enterprise, the test vehicle which was the prototype for Columbia, originally had the same wing markings as Columbia but with white chines and the "USA" letters on the right-wing spaced closer together. Enterprises markings were modified to match Challenger in 1983. The name of the orbiter was originally placed on the payload bay doors much like Enterprise but was placed