movies. All motion picture camera negatives are now shot on acetate film because it is safer than nitrate but not as strong as polyester bases, which may damage the camera rather than the film should a jam occur. Acetate can also be spliced with film cement, while polyester can only be spliced with tape or an ultrasonic splicer, so polyester is hard to edit. Acetate film does not burn under intense heat, but rather melts, causing a bubbling burn-out effect — this can be seen simulated in films such as Persona (1966), Two-Lane Blacktop (1971) ,The Muppet Movie (1979) or Velvet Goldmine (1998). It can happen during a film screening when a frame becomes stuck in the projector's film gate. Acetate films are also subject to degradation over time. With exposure to heat, moisture, or acids, the acetyl groups are broken from their molecular bonds to the cellulose. The now free acetic acid is released into the air. Acetic acid is vinegar, and thus the characteristic smell of vinegar is detected. This is known as vinegar syndrome. As the degradation progresses the film base becomes brittle and shrinks. Polyester In the literature of photography polyester is polyethylene terephthalate, which Kodak trade-named "ESTAR". Polyester is the most recent film base to have been developed. It was first used for specialized photography applications in 1955, but it was only in the 1990s that it became overwhelmingly popular for motion picture prints. It is highly preferable for post-production, exhibition, and archival purposes because of its flexibility, strength, and stability. Its strength is sometimes also seen as a disadvantage, however, in that polyester-base films are so resistant to breakage that they are often more likely to break the film equipment should a jam, or extra tension, occur. Movie cameras therefore do not use this base for shooting the original camera negative, as it is vastly preferable, and less costly in time and money, for the film to break instead of the camera. In addition, cameras require a lap-spliceable triacetate base to allow the use of all of the negative stock. PET film base can be ultrasonically spliced or by melting and joining the ends of two films together. Identifying a film base There are several factors which can aid in identification of the film base of a roll of film. Many are not 100% conclusive, and it is best to use a selection of