As a blacksmith, knowing when to take out the material from the forge is crucial. There is no way you can become a good smith without that knowledge. It took me quite some time to master this, so don’t expect to nail it the first time.
There are four common ways to measure forge and steel temperature for blacksmithing:
- Simply observing the color of the steel
- By using a magnet
- By using pyrometer
- By using a thermocouple
Each of them has its pros and cons, so don’t limit yourself to only one option. Now let’s examine each of them and find out what will work best for you.
Observing the Color of the Steel
This is the easiest and also the cheapest method of all. You don’t need any special equipment but your eyes. In old times, this was the only method for observing the temperature of the steel. Every smith had to rely on their eyes to guess the metal temperature.
If you are a beginner, I would recommend experimenting with different colors to see the difference between metal temperatures. For example, hammering the metal at about 1100°F (dull red color) is quite different than working at 2000°F (bright yellow color). The colder the metal, the harder it is to work with.
So, you can start with a dull red color. Make a few hammer strikes, put it back in the forge, and turn the heat up. Bring it to the orange color, take it out, and work on it. You should already feel the difference. The higher the temperature, the easier it is and the more time you have to work on it before it cools down.
Finally, bring it to the yellow-white color. Typically, this is the highest temperature you are going to achieve, depending on what type of forge you have. If you are using the coal forge, there won’t be a problem but if you have a propane forge, the story may be different. Namely, if your propane forge is not properly insulated or installed, chances are you are not going to achieve extremely high temperatures.
Of course, this method has its disadvantages. One of them is that it is not suitable for beginner blacksmiths. As I said before, it takes some time to get things right but as with anything else, the experience comes with practice.
The steel color on the outside is not the same as the inside. Keep in mind that your workshop should be darker. That way you can better distinguish the colors than you would do outside. This was my biggest problem in the beginning when I worked outside.
As you can see, forging temperatures range from 1100°F to 2000°F depending on the steel used. However, for the tempering process required temperatures are somewhere between 400°F and 600°F.
Tempering is the slow reheating of the hardened steel to achieve the desired balance of hardness and ductility. I should note that this process should immediately proceed after quenching to minimize the chances of cracking. Some of the benefits of the tempering process are reduced brittleness, better corrosion resistance, and increased ductility.
Colors range from bright yellow all up to blue color, depending on the temperature. Down below we presented a tempering color chart so you can know the exact temperature at which color corresponds to a particular color.
Using a Magnet
Except for observing the colors of the steel, using a magnet is another popular method for measuring the steel temperature. It is especially common among bladesmiths who heat treat low alloy steels with a torch or in a forge without any measuring equipment.
Namely, a magnet is used to determine the critical temperature at which steel becomes non-magnetic and it is ready to be quenched. While this method is not the most precise, it still gives you a good temperature estimation. All you do is you take out the steel from the forge and try to stick the magnet onto it. If it does stick, put the steel back in the forge and heat it more.
There are two fundamental ways of steel becoming nonmagnetic:
- By reaching the Curie point
- Transformation to austenite
Keep in mind that even if the steel loses its magnetism, you should put it back in the forge for a short period, usually about a minute or two. You don’t want to overheat it as it can ruin the whole heat-treating process.
Austenite transformation is a more common reason for becoming nonmagnetic with carbon steels, especially for those heat-treated with a simple forge or propane torch without sufficient temperature control. Know that some alloys like chromium tend to raise austenite transformation temperatures so steel becomes nonmagnetic noticeably below its standard austenitizing temperature.
Some steels require a precise heat-treating process while others simply don’t. So, if you are a beginner working on some simple projects, using a magnet to determine a steel temperature is a good reference point. On the other side, if you are an expert working with some unforgiving steel types, I strongly recommend using a heat-treating furnace. That type of furnace allows you more consistency of the heat. You can also dial the temperature and time necessary for the best properties.
One thing to keep in mind is that some steels require much higher critical temperatures than others. Always check the individual properties of the steel and its guidance before starting.
Pyrometer
Also called an Infrared thermometer, the pyrometer is a device used to detect the surface temperature of some object. In this case, it is steel. It acts as a photodetector due to its property of absorbing energy and measuring the intensity of EM waves.
They are available in a variety of spectral ranges. Mainly based on the spectral range, they are classified into single-color pyrometers, two-color pyrometers, and high-speed pyrometers. Pyrometers work on the basic principle of measuring the object’s temperature by sensing the radiation emitted from that object without being in contact with it.
Optical systems and detectors are two primary components of every pyrometer that are used for temperature measuring. First, the optical system captures the energy emitted from a particular object. After that, the radiation is transferred to the detector which of course detects the temperature.
Pyrometers are generally divided into two types:
- Optical pyrometers
- Infrared pyrometers
Optical pyrometers are used to recognize the thermal radiation of the visible spectrum. The high temperatures of objects mainly depend on the visible light they emit. Also, this type of pyrometer provides a nice visual comparison between the targeted object’s surface and a calibrated light source.
Advantages of optical pyrometers
- Useful for very high temperatures
- Good for monitoring moving object
- Good for monitoring a distant object
- It has good accuracy
- Doesn’t require any contact with the measuring object
- Lightweight
Disadvantages of optical pyrometers
- May be affected by smoke, dust, and thermal radiation
- Expensive
Infrared pyrometers are primarily designed to effectively detect thermal radiation in the infrared region. Infrared pyrometers measure the object’s temperature from the emitted radiation. This radiation is usually directed to a thermocouple to convert into electrical signals.
Advantages of infrared pyrometers
- Measurements can be easily taken from a distance
- Good for a variety of applications
- Advanced and memory measurement functionality available
- Simple to use
- Lightweight
Disadvantages of infrared pyrometers
- Requires clean environment
- May be affected by dust, high humidity, and smoke
- The accuracy may be marginal, depending on the model
- Can be expensive
For blacksmithing purposes, do not buy a pyrometer that has a maximum temperature of less than 2000°F as it will get destroyed in the forge. Unfortunately, I made that mistake. I bought a cheap pyrometer which was destroyed immediately in the first forging session. Don’t make that mistake people.
Since temperatures in the forge for blacksmithing purposes often go beyond that level, you should get one that can measure all up to 2400°F. I strongly recommend this BTMETER BT-1800 Non-Contact Pyrometer. For now, I used it for over 100 forging sessions and could not be happier than I am.
And now, let’s move on to the next and final option.
Thermocouple
Knowing what is going on in the forge is crucial at all times. The last thing you want is to overheat the metal and ruin the whole process. That is where the thermocouple comes in.
A thermocouple is a device that consisted of two wire legs made from metal. Those legs are welded together at one end, creating a junction which is where the temperature is measured. Voltage is created when the junction experiences a temperature change.
The primary difference between thermocouples and pyrometers is that thermocouples must be in contact with the measuring object while the latter doesn’t.
Note that there are many different types of thermocouples. Each of them has its own unique properties in terms of durability, vibration resistance, temperature range, chemical resistance, and many others.
For blacksmithing purposes, type K is one of the best choices used for measuring temperatures in the forge. Type K thermocouple is the most common among all types. It is accurate, reliable, inexpensive, and most importantly, has a wide temperature range. It can measure temperatures as high as 2300°F which is sufficient for most of the blacksmithing work.
My number one choice is Amprobe Thermocouple K-type. I have personally tried it and it works like a charm. Just last week my brother ordered it and has only words of praise.