Fire

Setting objects on fire can be tricky, especially when they can't cause harm to the building or when the sets must be able to be used for multiple days of filming. To cause a building to be on fire without actually burning it boxes are constructed behind the windows and used to contain smoke, flames and flares. From the outside it appears as if there is raging fire withing the building.

Differing types of flame bars are used to create fire effects. These are metal tubes with holes in a variety of places to allow the gas pumped into them to escape and be ignited. Propane gas is used for interior scenes and diesel for outdoor.

Another option for lighting a set on fire is to spread a highly inflammable gel on the objects, which lasts for a limited period of time.

Sets that must be caught on fire are mainly built with fireproof materials so as to ensure that only the gel or the gas use is what is burning. This also preserves the sets so they may be used across multiple days of filming.

Frame rate and Camera Lens

To capture the quick explosions they are filmed at a high frame rate. This allows the footage to be slowed down so that individual explosions that are detonated tenths of a second apart can appear as if they happen seconds apart. Likewise a long lens is used to make is appear as if a stuntman is closer to the explosions then he actually is.

No matter what type of explosion it is the safety of all persons present that must take precedence. Each explosion is carefully timed and tested and then choreographed along with smoke and debris to create the perfect look and the safest possible environment for the actors, stuntmen and personnel.

Underwater Explosions

While explosions anywhere are dangerous, they become increasingly more dangerous when they are underwater. On land it is able to mark where the explosions are so they are visible to the cast and crew, but when the explosive devises are under water, it is much more difficult to see them. While cork may be set to float about a devise that does not guarantee that the actors or stuntmen will see them, so it is up to the detonator to determine if the actor is at a safe distance from the explosive.

Large amounts of water have to be moved to create explosions under water, so industrial high explosives are used rather then black powder, which is not strong enough to create the effect. They have the be securely fastened to the bed of the river or sea and must be kept at the right depth. If it is too far under water, not enough water will be thrown into the air, if it is too shallow, the explosion will blast the water too far. Due to these factors and the need to have the devices within inches of the correct depth, it is important to have a huge knowledge of the local wave patterns.

Explosions under water create a shock wave which travels with much greater force then it would in the air. So it is imperative that personnel and other individuals won't be within the shock wave of the blast. Often small explosions or ultrasonic equipment is used to scare away local sea life to protect them from the shock wave.

Another option to create an underwater explosion is to use an air mortar. This is often used if it is too dangerous to use the high explosives or when filming in a studio tank. Air mortars have reservoir of compressed air that is released when a valve is opened.

Buildings and Cars

Movies often require a building to be blown up. The type of explosions described previously do not have the ability to blow up a building themselves, so pyrotechnics are able to make it appear as if a building is being demolished without actually hurting the structure. There are some cases though when the destruction of a building is needed, such as the hospital in Dark Knight. For these types there are two parts to the explosion, the "cosmetic" and the damage inflicting.

The structure is first weakened to the point where it is barely standing. Explosives are then placed at the remaining support beams. A highly explosive plastic like chord known as Cortex, Prima-Cord or Det-Cord is wrapped around the poles and set to go off at the same time as the "cosmetic" explosions. For the final effect, when the Det-Cord takes out the remaining structural supports, it appears as if the black powder and gasoline devises, that make the explosion look dramatic, caused the building to fall.

Car explosions are done in a similar way. They are gutted to take out all fuel tanks, lines and often times the engine. The doors and hood are taken off and reattached with cable. Often another cable will prevent them from flying off too far, restricting their movement. Multiple charges are then placed to simulate the way an actual car would explode, beginning in the fuel tank. Black powder mortars are used to blast off doors and such while gasoline fire balls give the impression of doing the work.

To cause a car to roll or flip over, an explosive canon devise is used. Part of a telephone pole is fitted to the car, with a black powder bomb above it. The stunt driver then detonates the bomb at the appropriate moment, causing pressure from the black powder to shoot the telephone pole out and propelling the car into the air.

Ingredients

Most explosions start with black powder. Composed of a variety of materials including charcoal and potassium nitrate, black power comes in a variety of forms, from powder to small chunks. Typically all it will do is burn when lit, but if placed in more compact conditions it can create quite an explosion. How it is prepared depends on how it will react. It is typically placed in plastic or cardboard containers; the tighter it is compacted and the thicker it is wrapped with tape the more violent the explosion will become.

By itself black powder does not create much of an interesting show, so different types of metal can be added to cause differing colors, much the same way fireworks are created. To give the effect of debris flying in the air soft materials such as peat, cement powder or pieces of foam are added on top of the black powder. Upon detonation these are then thrown into the air; caution must be taken to prevent any stones or hard objects from being placed among the debris to prevent them from turn into a potential bullet if propelled through the air.

To create explosions with fireballs a variety of liquid fuel is used. Most commonly used is regular gasoline, which makes an orange flame. Diesel is used to create a redder flame with black smoke on the fringes. Both of these continue to burn after the initial explosion and may fall on surrounding areas and continue to burn. Slightly safer is naphthalene and isopropanol which burns and finish after one explosion. Naphthalene gives a look between regular gasoline and diesel. Isopropanol is pure alcohol and creates a cooler flame that can be used indoors and closer to actors.

All of these ingredients are placed in mortars, which are steel tubes or pans in a variety of shapes that help direct the blast. While a round tube will blast objects straight into the air, a wider pan will spread the explosion outward. The black powder is placed on the bottom with a detonator chord attached to it. Next a bag of fuel is placed in the mortar with any debris material placed on top of that. Last is a second detonator at the top of the mortar to ensure that the fuel catches on fire.

Introduction

Pyrotechnics is the "art and science of creating fire and explosions". Explosions are made with very dangerous material and must be handled by a professional. Due to their dangerous nature every safety precaution necessary must be taken to ensure avoiding any harm to actors, stuntmen, personnel and property. The pyrotechnic supervisors have the final word in determining if a stunt required is too dangerous to be shot, and may refuse to do it. With explosions and stunts that can be done they are rigorously planned out to make them as safe as possible.