A discussion of the different types of air blast systems used on blacksmith's coal forges, and firepots or methods of supporting the fire, and styles of hearths. March 19th, 2001.

The air blast

Bellows

Under construction. This page is undergoing a large rewrite (as are most of the Forge Design series) and more material will be added and pages will be cleaned up for easier reading and faster downloading as I have time to make the changes. This page in particular is the subject of a very large addition to come soon. I am out of time so writing is stalled.

Pair of bellows

The most common style bellows for medieval blacksmiths forges that we find in books and manuscripts, is the pair of single-acting bellows or pair of bellows as I will describe them in this text. At left is a thumbnail photo link (click on it) to a 16th century woodcut depicting Emperor Maximillian visiting the armoursmith Seusenhofer's workshop, from Charles FFoulkes book The Armourer And His Craft, printed by Dover Books ISBN 0-486-25851-3.

Pair of bellows in use

Rocker shaft

Pair of bellows section under construction.

The Great Bellows

Towards the 18th century a double acting bellows called a great bellows or compound bellows, came into use. Combining the pair of bellows into a single great bellows with an upper and lower chamber saved a large amount of space in the shop

The upper chamber of the double acting compound style bellows (great bellows) supplied the air blast to the fire in a steady or constant velocity.  Pressure of the blast air and velocity of the air into the fire was adjusted by adding or removing weights placed on top of upper chamber board or hung from hooks attached to the upper board. Air is trapped in the upper chamber by a set of leather and wood flapper style check valves that are mounted on top of the middle bellows board. Once trapped in the upper chamber, the air could only exit through the nozzle hole carved through the wooden snout of the bellows, and then through the air delivery piping to the tuyere and into the fire. Leather sides and a leather hinge attaching the upper chamber board to the nozzle allows the upper board to rise and fall in relation to the volume of air trapped inside the upper chamber. A wooden spreader or rib is riveted to the inside of the leather chamber wall and located mid-way between upper chamber board and middle bellows board. The rib improves efficiency by preventing the leather sides of the bellows from expanding outward while the upper bellows chamber collapsed during used.

The lower chamber of the compound bellows forced air into the upper chamber when the smith pulled the lever, and check valves inside the bellows allowed them to trap air in proper directions. The bellows is quite large and takes up a substantial amount of room in the shop. To conserve space, the bellows was often mounted overhead. Still this takes up some substantial amount of room so often the bellows was mounted above work benches or other areas which didn't need much overhead room.

Detailed plans for building a great bellows can be found in the book The Blacksmith ~ Ironworker and Farrier by Aldron A Watson ISBN 0-393-30683-6.

The bellows began to be displaced as the sole source of air blast during the 19th century when mass produced cast iron hand cranked and line shaft driven blowers were manufactured. Today we use both modern electric blowers and old hand cranked blowers to provide the air blast for our fires.

The centrifugal blower

The blower moves air with centrifugal force by spinning at a high speed and pushing air outward into the fan housing and into the air delivery pipe. A series of gears or pulleys is used to step up the number of turns or rotations the fan wheel makes for each turn of the crank. The blower case around a good high volume machine is about 10 or more inches in diameter. Although some good blower fan cases are smaller, most smaller blowers are too inefficient for blacksmith's use. A good blower needs only about 1/2 revolution of the hand crank per second to force a strong blast in the fire. Most blowers have some type of mounting flange or lugs cast into their body or housing to allow mounting to a frame or stand.

There are two types of blowers to discuss here. One is Omni-directional rotation and the other is single direction of rotation. The Omni-directional blowers allow the smith to turn the crank both clockwise and counter-clockwise for full efficient blast of air. The cases of these types of blowers pictured above left, swell to a tear drop shape at the air delivery connection. What Photo? Photo missing?

At right is an example of the type of blowers that were meant to be cranked in one direction only. Note the difference in the shape of the fan case. The fan must be rotated in one direction for most efficient air blast and moves less air if turned the wrong direction.

Note that the style of crank rotation really doesn't matter except that the smith needs to know which way the blower is turned if he/she wants to use it or is testing one at a sale.

Soon after I began working as a blacksmith, I bought a new British Alcosa F-70 hand cranked blower (photo at right) imported from England. These were very expensive both because of importation costs and because cast iron is now very expensive to produce. Modern blowers may still be available, but due to high cost and lack of popularity they are no longer sold by Centaur Forge. Smiths who attend farm auctions and heavy horse auctions can still find good antique hand cranked blowers for sale.

Inspect Used blowers before purchase!

When buying a used blower at a sale, the first thing to inspect is the ease with which the crank can be turned and anything that suggests damaged or broken gears or bearings which support the gears inside the blower. Grinding noises or a rough grinding or grating feel of the crank in operation is reason enough to look for another blower. Check to see if air is being delivered by the fan. Check for clogs such as bird nests, hornets nests or other debris, damaged or loose gearing or fan blade attachment, and missing or broken fan blades. If the fan will not turn, check to see if the fan hub came loose and is binding against the fan case, or if debris is blocking rotation of the fan.

If any of the following conditions appear then the blower is junk:

Note: It is normal for blowers that are vented to open air to turn with more difficulty compared with those that are connected to a tuyere pipe of a forge! This is the opposite of what most newbie smiths would expect. Want an easy demonstration for yourself? Try blocking the output hole of the blower with your hand and then crank it, and then unblock the output hole and crank it again. The blower will be easier to crank while blocking the output hole and more difficult suddenly when you take your hand off the output hole. A resistance to air flow reduces the amount of work done by the blower so the crank becomes easier to turn when the blower is later hooked up to a forge.

Look at the condition of the mounting brackets to see if they are still serviceable.  If the brackets are broken off, you will need to build a frame to clamp the blower body into, and the supporting frame must allow you to oil and maintain the blower. Some blowers were supplied with stands and very likely still have them since the owners would have bought them originally to service an existing permanent forge. Other blowers may be sold alone and this occurs if they were separated from a forge that they had originally been mounted. And still other blowers may be sold with their original forge. If a blower is being sold with a portable forge, look closely to see if it actually fits the forge mounts provided. If the blower doesn't appear to mount easily to the forge, then it might have come from a different source and will need a mounting adapter or its own stand to be used with that forge. Don't let paint fool you, many sellers will gladly paint something to make it look 'new' and bring more money. Paint is cheap, broken blowers aren't.

Most blowers have cast fan blade hubs with sheet metal blades and missing blades can be replaceable. However to remain balanced if one blade is replaced, all must be replaced, and using a single method of attachment. Bent handle iron can be straightened with little trouble and broken or worn out wood hand-crank grips can be replaced.

The gear box of the hand cranked blower is filled with oil to the level of the drain plug. The drain plug is mounted slightly up one side of the gear case so static oil leaks will not seep so readily. To drain or change oil, the blower is tilted towards the oil drain plug hole and allowed to drain. Any method of mounting the blower must take into account that at some point in time the oil may need changed and the blower will need to be moved  to drain the oil. Reasons for changing oil might include using different weights for summer and winter use, especially if the blower is used in an unheated shop. After draining the old oil from the gear case, the blower is raised level and it should be noted that the drain plug is now located slightly up to one side as in the photos above. With the blower level, the drain plug is left uninstalled (or should be removed at this time) and oil is added through the top oil fill hole until oil just starts to run out the drain plug hole. The plugs are then reinstalled and the blower is ready for service.

Restoring an antique blower.

I recommend this route for any blacksmith with the time. Many blowers need only thorough cleaning inside to remove dried oil sludge and hardened deposits from the teeth of the gearing and from the gear cases. By scraping with an awl or screw driver, the hardened deposits can be removed from the inside of gear teeth. Be careful here though, some blowers used one or more fiber or cellulose gears to quiet the blower during use. Scraping will damage the fiber gear teeth, so be gentle with the fiber gears. Alternate soaking in solvent and scraping to help loosen deposits. Scrape all deposits off the inside gear case and bearing supports as well. Thoroughly scrap and clean the seal seats so the gear case can be counted on not to leak when reassembled. When cleaning is finished use oil or thin white grease to  lubricate bearings and gears before final assembly. Seal areas must be free of all grease and lubricants before final assembly. When all is scraped as much as you can, run through a parts washer machine.

New wooden crank-handle grips can be made by a local wood turner or anyone with access to a wood lathe. Custom turned handles are much more comfortable and the turner can make better shapes than what came with the blower originally. See the photo at left. The Alcosa F-70 blower at far left side of the photo above, was bought new 20 years ago and always stored indoors, so the handle is in good condition. The blower on the right side of the photo was bought as an antique and the handle had rotted off. A new custom made handle grip was installed. At one time I used to try to replace broken and rotted wooden hand-crank grips with new ones I fashioned from file handles. I now advise against this as wooden file handles are very uncomfortable to use and work poorly.

Bent crank handles can be straightened with little effort. The wooden handle grip is held on to the crank with a special retaining washer that is riveted onto the end of the crank. Using a hammer and punch, and carefully working the riveted section of the crank which holds the washer in place, the iron can be upset inward enough to allow the washer to slide off the end. The wooden hand grip is thus free to be removed or installed. The special retaining washer has a small outside diameter so to fit inside the recessed hole inside the wooden handle grip. A regular washer between the inside of crank and the handle grip help keep the grip from chafing against the inside curve of the handle. With the wooden handle grip and its retaining washer in place, the end of the crank handle is peened out over the hole (clinched over the sides of hole) in the retaining washer until the smith believes the washer won't come off again. A recess in the end of the wooden grip keeps the sharp edges of the retaining washer away from the hand when operating the blower. This recess is drilled into the outside end of the new wood grip before assembly.

At left are a couple of photos of a centrifugal blower sent to me by Charles A. Derrick. In this case the blower is driven by a series of belts and pulleys rather than the enclosed gearing seen on the blowers above. While this example looks different than geared blowers, the principle of operation is identical.

The pulleys increase or multiply the speed of the impeller as the smith cranks the handle, in a series of two steps. The first belt fastened from the top large pulley to the small pulley below it. The small pulley is mounted permanently to the large lower pulley. A belt fastened around the large lower pulley to the small pulley on the back of the blower completes this link to the blower. The speed increase as a result of this stepping of pulleys is comparable to geared blowers.

Bearings for the pulley axles are Babbitt. There should appear an oil hole on top of or near each bearing where the axle enters it to, allow oil to flow into the bearing by gravity.

The belt driven blowers like this one were more common around the mid 19th century while the geared blowers came into use around the late 19th and early 20th centuries.

The good versus the bad

The good blacksmith blowers all have one thing in common; large impeller blades. Measured from where they fasten to the impeller hub, to the ends farthest away from the hub.  This length is very important as it is these extra long paddles that produce the high pressures needed to force air through the fire, and also to overcome back pressure and resistance caused by the bends in the delivery piping and tuyere. Most good blowers are approximately 12 inches in diameter. I have had one in the past which was about 9 inches in diameter which worked just as well but this one had impeller blades of equal length to the larger blowers, which were simply mounted closer to the center of the impeller hub, and the blower was geared to spin much faster than a larger diameter blower to compensate for the smaller diameter.

Squirrel cage blowers such as those found on oil burning or gas burning boilers and furnaces, heating and air-conditioning systems, or even hair dryers, will not produce enough pressure to overcome all the resistances in the blacksmith's fire. For those new smiths who think they will just use a oil burning furnace blower or hair dryer, I want save you the effort right here. Don't bother. Instead get a real blower for forge use. Squirrel cage blowers won't develop enough pressure no matter what diameter they are as the length of the blades is very short, (measured radial) measured from hub shaft to outer tip of blades. Again it the long length of the paddles or blades in the blower which create the higher pressures needed to force air into the blacksmith's fire. Take this advice from one who has been there. All of the blowers seen on this page are good blowers. They are worth the effort to find if the smith wants to build a hand powered forge. I know that many blacksmith suppliers sell squirrel cage blowers for use on forges. That does not mean that they have some special brand that works. I used one belonging to a friend and found out first hand that these little blowers do not perform well enough. While they do blow some air into the fire, they are not strong enough to make the fire hot, and the air blast is weak enough that the fire soon packs with fines as a result of a blast that was too weak to burn the fines away.  Again, save the time and money and buy a real forge blower.

The Fire Pot

The interior of the firepot is shaped like a bowl and is either round or square overall shape. Like a bowl, the firepot tapers inwards towards the bottom, and is 2 to 6 inches deep with 2 - 4 inches being the most practical depth. The understand why blacksmith's prefer a firepot depth of no more than 2 - 4 inches for heating long bars, see Using the Fire at; http://www.beautifuliron.com/usingthe.htm. Built into the bottom of the firepot is an opening for the air blast to enter the fire, and a tuyere or a means for attaching one, and a special fixture called a clinker breaker. Photo at left, the firepot at bottom right is a Centaur Vulcan with a dumping ash gate. The firepot at top right has a sliding leaf which allows the length of the fire to be adjusted. Bottom left is a ducks nest for use in building forges which have no firepot but in which a cast iron tuyere is used and a the forge hearth is filled with rammed clay to form a depression for the fire. At top left is the firepot supplied with a ready made forge by Canedy-Otto.

The clinker breaker fits inside the air blast hole in the firepot and supports the coke from falling through the air blast hole, at the same time, many clinker breakers are specially designed to direct air flow into the fire in a very advantageous path for creating very hot fires. Some clinker breakers are a solid triangular or elliptical shape while others are hollow and slotted to allow some air blast to actually pass through them as well as around them into the fire. I found neither style to be better than the other so this is probably more a matter of opinion as to which is better. As the name 'clinker breaker' suggests, the clinker breaker is used to loosen clinkers prior to cleaning the fire. Clinker flows to the bottom of the fire, and by rotating the handle of the clinker breaker, the oblong shape of the clinker breaker lifts the clinker and loosens it for easier retrieval from the fire. 

Around the outside of the firepot is a large rim which allows the firepot to be placed upon the forge hearth. The hearth is cut out to allow the firepot to fit and the firepot rests on its rim inside this cut out area. The Vulcan style or square firepots were made for use in the old masonry forges that were very popular in the United States during the early 1900's. Those old forges had a trough running across the hearth which helped the smith build a high fire and still place long bars through the heart of the fire. For an example of one of these old style masonry forges see my Quasdorf Wagon Shop forge page in Dows Iowa, http://www.beautifuliron.com/dows.htm. The front and rear edges of the Vulcan style firepot were lower to place them down in this trough in the masonry forges. For anyone building a simple steel forge without a trough, a carefully planned set of shims welded to the hearth where the sides of the firepot will come to rest, will support the firepot at a height that allows the front and rear edges of the firepot to be level with the hearth. For an example of how to do this, see the bottom of the page Steel Side draft Forge at http://www.beautifuliron.com/mysteel.htm. To create the trough in the newer style steel forges, several rows of firebrick are placed along the sides of the firepot. 

Ash gates cover the bottom of the tuyere of a firepot. Since fines cinders fall through the air hole in the bottom of the firepot over time, a means of removing them periodically from the bottom of the tuyere is needed. The ash gate allows fines and clinkers to be removed from the bottom of the tuyere, afterwards the gate is closed so air is forced to flow into the firepot when the forge is being used.

The best firepots are made of heavy cast iron and come supplied with a cast iron tuyere, clinker breaker, and ash gate. Cast iron resists the intense heat of the blacksmith's fire without excessive oxidation (rust and scale). The firepot makes fire tending much simpler and creates an environment in which much hotter fires are obtained with less effort. The Centaur Vulcan firepot photo at left (second from left), is a good example of a good blacksmith's firepot, 4 inches deep, high side rims for use in the old style masonry forges with troughs, heavy cast iron, a good tuyere and clinker breaker, and choice of sliding and dumping ash gate (the dumping ash gate being more convenient). The Centaur Horseshoer's firepot is another good choice, slightly less deep at about 2-1/2 inches deep, same tuyere and ash gates as used in the Vulcan firepot, and a round shape and rim. Both available through Centaur Forge (see my Links page at http://www.beautifuliron.com/links.htm)

Good cast iron firepots last a long time. Mine is 20 years old. But they do have one weakness which must be avoided. Never pour water on a hot firepot. There is no reason to do this but amateurs often use excessive water on the fire. When hot firepot comes in contact with water, it will crack or break! Don't pour water on a hot firepot.

What did blacksmiths do before firepots came into use?

Blacksmiths used sideblast style forges before firepots became available in the United States. Long after firepots became available, many smiths continued to build their own sideblast style forges. In some countries today smiths continue to build sideblast forges. England is a good example. Manufacturers there continue to make modern sideblast tuyeres available. And I will begin experimenting with a sideblast style tuyere soon because I suspect that it allows more welding to be done before the fire clinkers up compared with a typical firepot. Today most smiths in the U.S. use firepots because they are convenient, very easy to use compared with other materials in forge building, and because the sideblast tuyere has not yet made a large appearance on this side of the ocean.

Blacksmith's built their hearths any way they thought best based on the types of construction materials available. Everything from a hole in the ground with a pair of goat skin bladder style bellows, to a large brick enclosed medieval oven shaped hearth with a flat hearth and a pair of tuyeres aimed into the side of the fire from the other side of the hearth wall. In Great Britain side blast forges are still in use made by Vaughns of England (they apparently bought British Alcosa),  large cast iron or fabricated hearths with a large hood and a rear wall with a cooling water tank and hollow cast iron tuyere fitted through it and into the fire. These are more complicated to use compared with an American firepot, but worked very well in the hands of an expert. 

In the 1800's before firepots came into use, the earlier cast iron forge appliances included the cast iron ducks nest tuyeres. These offered the smith the ability to quickly maintain a forge hearth made of rammed clay and added the fast clean out of the tuyere with the addition of the ash gate. Some even had clinker breaker and focused the direction of the air blast just like modern firepot tuyeres do now.

The Tuyere

A tuyere is the air blast pipe or plumbing which directs air from the bellows or blower, into the fire. The tuyere is the air pipe coming directly in contact with the fire. Good firepots come with a tuyere already assembled onto them. In the old side blast forges that used bellows, a pipe or pair of pipes from the bellows directed air into the side of the fire. The pipes were most likely made of ceramic or stoneware materials used by the local potter, or were made of clay rammed around some type of core or pattern mold. Ash and debris in the tuyeres of side blast forges isn't problem since it would simply be blown back into the fire upon filling the bellows.

New side blast tuyeres in British Alcosa forges are cast iron and bolt onto the water tank on the back of the forge. A set of gasket seals around the tuyere through the front and rear of the water tank, makes the connection water tight and allows the water in the tank to circulate through the tuyere to cool it. The air supply pipe fits into the open rear of the tuyere.

The tuyere on the firepot is bolted to the bottom of the firepot. It delivers air to the bottom of the fire and has sump in the bottom to catch cinders that fall through the clinker breaker and keep them out of the way of the air blast. A gate or valve built onto the bottom of the sump allows the sump to be emptied periodically and is then closed to block air blast from escaping through the opening. There are two styles of ash gates. One is a sliding ash gate, held on with a single bolt or screw. By rotating the sliding gate, the ash falls out the bottom of the sump. Friction keeps the sliding ash gate in place. The other type of ash gate is a dumping ash gate. The dump gate pivots on a single bolt and is held closed by a counter weight built onto it  opposite the gate valve. The dumping ash gate is far more convenient and comfortable to use. but both work well.

In the area just beneath the flange mount of the firepot (just below the firepot) a hole is drilled through both sides of the tuyere for the clinker breaker pivot rod. A clinker breaker is then fitted in the middle of the air blast hole between the tuyere and firepot, and the pivot rod is inserted through the it and the drilled holes in the tuyere. The clinker breaker is secured to the pivot rod with a bolt or screw and since it is exposed to intense heat, the screw will seize to the rod and cannot be removed easily later on. Forge builders must plan on this since moving the firepot to a new forge may be hindered by awkward placement of the pivot rod. The short pivot rods supplied with these firepots is a good length to work with.

Updated December 23, 2007.

This page is still under construction and more to be added.

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Page created April 2000.