The HEX Fireworks - 'Knowledge of Fountains'
Fountains – By Christopher Adlam, General Manager, HEX Fireworks and previously Factory Manager, Kimbolton Fireworks Ltd.
A pyrotechnic fountain is a firework that produces a spark effect from a thick paper tube or cone.
When we discuss fountains in the professional firework industry, we often refer to them as gerbs, an old phrase derived from the French word gerbe which means a spray or a sheaf of corn. I started working in the factory at Kimbolton Fireworks 20 years ago and they were always referred to as gerbs during my time there. These fountains were straight walled tubes, for professional use rather than ‘cones’ which are predominately sold in the consumer market.
Straight walled gerbs are either designed to produce powerful sprays or a more delicate squat effect. The effect will normally last 25-30 seconds. They can be made with various materials to produce the different type and colour of sparks. Charcoal from different trees and various sizes of granule are used to obtain rich, deep golds. Unless a large gerb is used, the sparks produced using this material are quite fine and shorter burning and therefore best observed at minimum safety distances. Aluminium is used to produce glitter and flitter effects; where you can see the sparks branching as they burn. Such gerbs are often overlooked in displays but are quite enchanting to watch. Sodium Oxalate produces yellow glitter which is best seen in slow-moving wheels where the sparks are allowed to move slowly in the air. Bright silver compositions use titanium of various grades. These can be quite large flakes or small granules called ‘sponge’. These large flakes are found in the largest gerbs where a bright spark is needed for the longest duration and the brightest effect.
At Kimbolton Fireworks Ltd, a 75mm internal diameter gerb used to be manufactured in batches of four items per pressing, resulting in each gerb containing approximately one kilogram of composition. The inside profile was curved to allow some delay between the initial ignition and the fire reaching the full burning surface of the composition. Originally, they were made so that the fire went through the choke hole and spread across the whole 75mm diameter surface straight away. In this instance they immediately reached full power, producing a 30 foot high column of sparks and a tremendous roar. It was both disconcerting and amusing for the production testers to see the owner of the company, the chemist, take a step back sometimes when the gerb fired. If he could feel the power in them and moved away, so did we! Hence the decision was made to change to a more rounded internal profile, to allow some air space to develop before the fire spread across the full composition burning surface.
A straight tube is preferred by professionals because once the fire transfers to the composition the fountain will almost immediately reach its full spark height. This is what the professional wants. The power is derived in the same way as a rocket motor, using the Venturi effect. (See the ‘It’s not ‘Rocket Science’…Well actually, this page is!’ for a more detailed explanation.) The end of the tube is constricted by means of a pressed clay choke, or mechanically pulled in using a crimping tool. For some compositions, such as glitter, a crimped tube is necessary because the burning composition produces an amount of molten slag (combustion by-products) which could potentially block a clay choke. The crimped choke of a paper tube will burn away slightly during its performance and allow the molten slag to leave the tube, and the sparks to continue to exit the tube unhindered. Sometimes it is necessary to add ‘first fire’ composition, which is slow burning and designed to burn past the choke and onto the main composition. The tube can be seen to relax as it is removed from the filling jig and a fierce, fast burning composition could potentially become loose and powdery at the choke hole. This could result in the tube exploding after ignition.
Gerbs of this type are becoming used less often in displays where the current favour is single ignition barrages (cakes), single shots and shells and, also in decreasing numbers, roman candles. Gerbs take time to arrange into a pattern and fuse ready for a display. However, when done correctly, with care and attention paid to spacing and set-up, they can be quite spectacular; another expression of the pyrotechnic art. When producing lattices of colour changing fountains (most commonly gold to silver) it is preferable to make sure the fountains are from the same batch. This ensures that the fountains all change together which is a delight to the audience and a point of pride for the pyrotechnician.
For the consumer, the most common form of fountain is a cone. This a style that is easy to fill so long as the composition is dry. The cone is inverted and filled from the bottom. Sometimes sawdust or some other inert material is added and then sealed with a strong paper disc. A delay fuse is added to the tip of the cone. Care must be taken however, because the composition can potentially be pushed down the cone during transportation and lead to cracking or loose powder. This in turn can lead to the fire flashing down through the compromised composition and bursting the tube. The composition must be dry before introduction so that there is no shrinkage inside the cone if it were to dry out once filled. Shrinkage leads to the same problem of fire flashing down the internal side of the cone which may lead to over pressure and the tube bursting.
The burning characteristics of a cone are quite different from that of a straight walled gerb. When a cone is lit the burning surface is small, this allows for the air chamber above to increase gradually accommodating the increasing pressure. When you watch a video of a cone fountain you will notice it will take some time to reach its full height which it will then maintain until the composition is consumed. Large consumer cones can have a variety of effects added to the composition, such as coloured chips and crackle stars. These coloured chips or ‘micro stars’ are made in such a way that they have a slight delay built into the structure. This allows time for them to leave the cone and display their colour before being consumed. Getting the balance between the size of the exit hole and the performance of the micro star is a challenge. The hole must be big enough to enable a moving micro star to leave the cone, but small enough to retain the necessary power to push out the micro star with enough force to get it to the desired height in the air.
Cones provide the consumer with some interesting ground fireworks and their place in a display should not be overlooked. Given some thought and planning, cones can be used in very creative ways and can emulate the lattices and patterns of the professional display.
Reference source: Rev. R Lancaster MBE, Fireworks, Principles and Practice, 4th Edition, ISBN 0-8206-0407-0