Gravity bong
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In "cannabis culture," gravity bong is a term that can refer to either of two devices used for smoking cannabis that use water to create a vacuum whereby the suction created draws air through a bowl to fill the container with smoke.
Gravity bongs differ from regular bongs in the fact that they usually do not filter the smoke through water, they have a far larger chamber, the smoke is much denser than possible in a standard bong, and they are usually homemade. Additionally, rather than the operator reducing the air pressure inside the device through inhalation, this is achieved with either falling water or by lifting the device itself while it is partially submerged in a bath of water.
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[edit] Terminology
Gravity bongs are named for gravity's role in their function. Upon using a gravity bong, suction is created when trying to restore the pressure equilibrium of the inside of the bottle with the outside air. Both devices described below are known as gravity bongs and not all people will be familiar with both versions; many may know their device as a gravity bong instead of using the more specific term for each device. While both methods share similarities that are critical to their operation, they differ in the technique that is used to control or constrain the rate of air flow through the bowl. Gravity bongs are prized for their efficiency and their low construction cost. A Gravity bong can be made with less than a dollar in supplies.
[edit] Bucket method
A gravity bong using a bucket can be constructed in many different ways, allowing for variations in material availability. In all cases, however, some sort of a bottle and a bucket are needed. The size of the bottle is very important to operation; a larger bottle is generally preferred. For best results, securing smaller bottles inside the large one prodcues a stronger vacuum while still optimizing the volume of the big bottle. The larger the bottle is, the more volume that is available to fill with smoke. The size of the bucket is contingent on the bottle; it is ideal that the bucket can, when filled with water, allow the entire bottle to be submerged.
The bottle is prepared by cutting the bottom of the bottle off, leaving it as tall as possible. Also some sort of bowl or stem is somehow placed in the bottle cap, usually by cutting a hole. It is absolutely vital to the proper operation of the gravity bong that the seal on the hole in the cap is air tight, such that the only air that can enter through the cap is through the bowl or stem. Also common is a small hole placed near the top of the bottle to be used as a shotgun or carb. The purpose of this carb is to allow clean air into the bottle from another source other than the cap, and to keep water from entering the mouth of the user if they inhale too forcefully. A simpler method is to remove the cap and place tin foil over the bottle top. The foil is shaped into a bowl shape, pierced with small holes, then filled with cannabis. Another method for making the bowl for a gravity bong is to use a two liter bottle, and use a knife to cut a large hole in the cap of the bottle. You can then use a salt or pepper shaker top for the actual bowl, giving you a lasting and safe bowl that works every time. Half of a capsule tea strainer is also particularly effective, especially since its mesh or holes are designed to be just small enough that shredded tea leaves (which are often the same size as shredded marijuana leaves) don't slip through. The one drawback to both bowls is that, owing to their considerable size (compared to glass stem bowls), matches or grill lighters are required to light them. When placing the cap onto the bottle, it is not necessary for the bowl to be screwed onto the lip of the bottle fully. If a sufficient and even hole has been made, there will be no need for sealers of any kind.
The bong is operated by submerging the bottle in the bucket, with only the open top of the bottle protruding from the surface of the water. The bowl is filled with a substance to be smoked and then screwed onto the top with the bottle still in the water. The bottle is then slowly lifted out of the water while the carb is being covered with a finger, at the same time as a flame is applied to the bowl. Some people prefer using a butane or "torch" lighter to further enhance combustion of the material, making the "hit" even more dense. The speed at which one pulls the bottle up depends on how dense of a hit is wanted, and how well the material is burning. Generally the rule of thumb is to pull fast enough that the material is burning and smoke is flowing into the chamber, but slow enough that the "hit" will be dense. The pressure will pull the flame into the bowl, burning the substance and pulling all of the smoke into the bottle. Once all of the material in the bowl has been burned or the bottle is full of smoke, the cap is removed, the user exhales all air out of the lungs, and then places his or her mouth over the opening. The bottle is then pushed down into the bucket, forcing the smoke into the user's lungs.
When the bottle is pulled out of the water with the carb covered, the air flow into the bottle is restricted to whatever is allowed through the bowl. Any air that enters the bottle must come through the burning material; thus the air is consumed during the combustion process and the only gas that enters the bottle is very dense smoke. Typically the amount of smoke entering the bottle is not enough to account for the change in volume which results from pulling the bottle out of the water. Thus to keep the pressure inside the bottle the same as the surrounding air the water level inside the bottle raises above the water level of the rest of the bucket.
The force of gravity's pull on the water inside of the bucket creates the suction that pulls air through the bowl. The force of the suction is directly proportional to the level that the water in the bucket is raised above the surrounding water level. The higher the water level is pulled creates a higher force due to gravity, because of a larger volume of water being suspended higher.
The pressure inside the bottle is governed by the equation 
, where p is the pressure of a fluid column, ρ (rho) is the density of the fluid, g is the gravitational acceleration, and h is the height of the fluid column. As the water levels in the bottle and bucket separate, the difference in height causes a pressure difference between the bottle and the atmosphere. A larger pressure difference means that air will be sucked through the bowl at a higher flow rate which is not always ideal. A high rate of air flow may result in the substance in the bowl burning faster than is ideal to maximize the amount of smoke. Another complication resultant from excessive air flow may be the substance is pulled through the opening of the bowl before it is fully burned, producing wasted material.
After the bottle is full of smoke, and all combustible material is burned, the user forces the smoke into their lungs by pushing the bottle down. The effect that is produced from pushing the bottle down is the opposite obtained from pulling it. When the bottle is pushed down the water level in the bottle is pushed lower than the top of the water, which results in the gravitational forces on the surrounding water to force the water level back to equilibrium in the bottle. To maintain equilibrium, the gas in the bottle is forced out the now open top into the lungs of the user at a higher force than usual. Alternately, some users prefer to suck on the mouth of the bottle, drawing both smoke and water upwards. If the bottle is equipped with a carb, the water level will only rise to the level of the opening. Otherwise, great care must be taken if the user does not wish to draw water into the mouth.
According to some, this bong maximizes the effect of the substance smoked like no other method can. This is due mainly to a number of factors, the enormous density and volume of the smoke, and the velocity at which the smoke is forced into the user's lungs. Therefore, gravity bongs are generally known for producing a very intense experience - one which a regular bong cannot. A gravity bong produces a hit that contains much more volume than one produced from normal methods, because the user usually stops before they've sucked in a gallon of smoke, however a gravity bong is a large bottle, completely filled, thus ensuring a maximum potential for a "huge hit". Critical to the large volume is the high density of the smoke which is produced due to the restricted source of air flow, only through the bowl. Combining these factors with gravity forcing the smoke into the user's lungs at an increased rate, the outcome is a method that surpasses any other in the amount of effect that produced from the smoked substance.
[edit] 3-Liter Variation
A variation to the bucket method is the use of a 2-liter inside a 3-liter soda bottle. The top is cut off the 3-liter, and the bottom is removed from the 2-liter bottle. A slide piece and bowl are inserted into the cap of the smaller bottle. The larger bottle holds the water, while the smaller bottle serves as the smoke chamber. Another variation would be securing 5 sizes of Gatorade bottles together, starting with the half gallon working down to the 16. oz. The device is operated in the same fashion as described above.
[edit] Soda Bottle Gravity Bong
Another form of a gravity bong is known as the "third lung". This is made by taking a two liter soda bottle, emptying the contents, and then cutting the bottom of the bottle off. A large ziploc bag is then placed over the bottom of the bottle and then rubberbanded in place. Then a pair of scissors is taken and pushed through the white top of the bottle, creating a small hole, just big enough for the bowl to be placed in. The ziploc is pushed inside the bottle, and then the herb is put into the bowl. The marijuana is lit, while the bag inside of the bottle is grabbed and slowly pulled out. The smoke is drawn inside the bottle and is ready for smoking.
[edit] Modifications
Instead of removing the bottom of your bottle, poke holes into it after heating with a flame. This allows you to remove the bottle entirely out of the water without any smoke escaping.
Optional bucket bong accessories include a bong water guard. A guard consists of a buoyant sphere, often a ping-pong ball, that is placed within the bottle before the hit is drawn. Slight modification of the bottle will keep the ball inside the bottle at all times. Instead of having the entire bottom of the bottle cut off, wedges of the bottom may be cut out. When flat, they will not allow the ball out, but when gently twisted the gap is large enough to allow pong ball access in and out of the bottle.
Another modification is a percolation tube that draws the smoke down into the water by connecting to the male slide piece. The bottle must be raised so that the mass of the volume of water inside the device above the water's surface can gravitationally pull the smoke through the percolation tube. If the tube is long or the device is not airtight, it will not work. This method produces a toke that is generally more potent and also decreases the damaging heating effect of the conventional gravity bong.[citation needed]
Another modification that is especially convenient, because of the lack of water, is the basic "jig." First, get a socket, around 3/16." Get a soda bottle cap, off of a 20 oz, 24 oz, or 2 Liter, and a pointed knife. Heat the point of the knife up with a lighter and take the knife and bore a hole in the cap into as perfect of a circle as you can. Make the hole slightly smaller than the bottom of the socket. Take the socket and jam it through the top of the cap (sometimes you have to push pretty hard). This will ensure air tightness and then you have a "jig cap" as long as you don't lose it. Finally get either an empty 20 oz, 24 oz, or 2 Liter bottle, and burn a hole in the bottom of it. Screw the jig cap on the bottle and light the marijuana, by pulling through the bottom of the bottle. Once the bottle is filled up with smoke exhale and then hold the remaining smoke in the bottle. Finally, take the cap off and clear the bottle.
[edit] Other vacuum methods
As with the waterfall method, the bucket method uses gravity to create a vacuum. Its claim for the title of "gravity bong" is supported mainly by the argument that it relies more on the force of gravity. The bucket method uses gravity's pull both to create a vacuum, and to control the air flow. When the bong is pulled using the bucket method, the air flow rate is directly proportional to gravity's force due to the difference between the water levels of the bottle and the surrounding bucket. Some argue that the bucket method in fact does not need gravity at all. All it needs is a device to create a vacuum; therefore it can also be referred to as an air displacement bong, or pressure flux bong. Gravity merely holds the water in place (making it resemble a liquid piston). If a solid piston is used instead of the "liquid" piston, the direction of gravity is irrelevant, much like in the piston operation of an internal combustion engine. Attaching a plastic bag to the end of interior plastic bottle of the bucket bong would serve as this solid piston, a bong commonly referred to as the third lung.
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[edit] Waterfall method
Similar in design to the bucket method is the fountain method, also known as a waterfall bong. A Fountain usually consists of a plastic bottle with a hole punched at the bottom and a removable bottle cap with a bowl in it, or an alternative to that is a guass, which is usually a piece of round metal that fits on the bottle with a mesh inside. The bottle cap or guass is removed, the hole is covered by a finger, the vessel is filled with water, then while holding a flame to the bowl, the finger is removed. Gravity causes the water to flow out creating a vacuum inside the chamber and smoke begins to flow into the chamber; see the section on the bucket bong for a more detailed description of this process. When all the water has run out the user removes the bottle cap or guass, places their mouth over the opening at the top of the bottle, and inhales the contents. A quick video guide to making one using a 1/4" socket and plastic bottle can be found at link. The colloquial term "fountain" is used frequently in Europe and Australia, waterfall is the American translation.
A waterfall bong can easily be made out of a glass SoBe bottle, resulting in a "SoBe bong".[1] These bottles have a thin spot near the bottom as an artifact of the manufacturing process, which can be punched out without destroying the bottle, creating a perfect drain hole. This form of waterfall bong is ideal because it is glass and that makes for a tastier smoke. A stem and bowl can be inserted through a hole drilled in the metal bottle cap, and the hole sealed with hot glue or electrical tape. However, a bowl is not necessary. The Sobe caps -even the coloured ones- are completely fire proof. You can poke small holes in the cap, remove and discard the rubber seal, and give the cap an indent to make a concave screen that can work just as well as a bowl.
[edit] Brewing
There are many ways to classify the smoke inside a gravity bong. If the smoke is white then it is known as a 'whitewall', and is usually smooth and creamy. If the smoke is yellow it is a 'yellow wall'; this is usually quite harsh on the lungs. The next step up is a 'brown wall', which is when the smoke near the top of the bottle is tinged a light shade of brown. This will leave an unpleasant taste in the mouth. The last stage is a 'golden wall'. This is when the smoke is slightly golden, and will leave an undesirable taste in the mouth for a duration of several minutes.
==Bucket Trick==
There is a way to pull the bottle out of the water that will make a small stream of smoke "fall" from the top of the bottle to the water. This is achieved by pulling slowly to begin and then speeding the pull and slowing down again. Once the right speed is achieved the trick can be preformed. If done correctly the smoke appers as a "waterfall" falling from the top to the bottom of the bucket.
[edit] Health Issues
If a gravity bong is constructed with potentially harmful materials such as inappropriate metals (i.e. aluminium) or poor construction resulting in plastics/glues and other materials which can be safe but are not intended to be heated, it is likely that the user will inhale fumes more dangerous to their health than the intended cannabis. On occasion other materials are used.
[edit] References and further reading
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