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What is Slag in Welding? (A Complete Guide to Weld Slag)


Welding slag is produced as a by-product of some arc welding processes, most commonly shielded metal arc welding (also known as stick welding or SMAW), electroslag welding, submerged arc welding, and flux-cored arc welding (also known as FCAW). Slag forms as a hardened layer of material on top of welds made using these processes and is created from the melted flux they use.

This makeup of this layer of slag differs according to products and processes used for the weld. It is formed when the flux – a solid shielding material used in the welding process – melts in or on the top of the weld zone. Thermodynamic processes in the molten weld pool cause any elements that are not included in the pool – including the flux - to be forced out to the surface where they solidify into slag.

This slag includes non-metallic elements like aluminium, silicon and calcium oxides as well as quantities of nitrogen, hydrogen and carbon. Welding flux is made of carbonate and silicate materials so that it can form a slag that protects the weld from oxidation and contamination in the weld. The melting flux releases its own gases that push atmospheric gases away and prevent unwanted oxidation or nitrogen reactions in the weld area.

As well as shielding the weld from contamination, the slag formed by melted flux can also be used to keep the weld pool in place and in the joint as it cools, which is especially important when welding out-of-position. The covering flux also thermally insulates the weld and reduces the cooling rate.

Welders use flux materials that have a lower density than the weld metal, so that it floats to the top of the weld pool once melted and leaves a pure (or nearly pure) metal to solidify below. Some flux materials can contribute to the behaviour of the molten metal to provide physical or chemical alterations.

The characteristics of the slag produced by both gas-shielded and self-shielded flux-cored wire depend upon the materials used in the flux. High levels of titanium oxide (TiO2) lead to a faster-freezing slag that helps with out-of-position welding, but too much titanium can make the slag sticky. Higher levels of silica in the flux can make the slag difficult to remove, while lime in the flux makes the slag easier to clean up.

However, for all the benefits that flux and weld slag provides, they can also create problems…


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Weld Slag Inclusions

If, for some reason, the slag doesn’t float to the top of the molten weld pool it can become embedded in the solidified metal, which creates a welding defect called inclusions

These weld slag inclusions can be visible on the surface of the weld or may be hidden in the metal and only detectable by X-raying the weld.

Inclusions can negatively impact the quality of the weld and may require drilling or grinding to remove before the section is re-welded.

Common causes of weld lag inclusion include using poor quality flux, handling the flux incorrectly or through poor welding technique.

How to Avoid Slag in Welding

Weld slag shouldn’t be avoided as it does have benefits (see above), but at the same time it is important to avoid weld slag inclusion, which detracts from the weld quality.

Using best practice techniques can minimise the risk of slag inclusion, but first it is important to select the right flux-cored wire for your application and use it according to the manufacturer’s recommendations. While assessing the flux wire it is worth investigating the slag that is associated with it, which will help inform you as to how much interpass and post-weld clean-up is required.

There are also a number of welding tips that can help reduce the chance of slag inclusion:

  • Remove Slag During Multi-Pass Welds: You can help prevent slag from becoming trapped inside a weld when performing multiple passes by fully removing any slag after each weld.
  • Welding Angles and Techniques: When performing horizontal and flat welds keep the gun angle between zero (straight up and down) and 10 degrees, as well as using a drag technique to minimise the chance of inclusion. When welding in a vertical-up position, you will need to alter your technique according to the gas or flux-cored wire being used. Wires that produce a heavy slag can still be drag welded in a vertical-up position. A zero degree angle will also produce good quality bead shapes with a T-1 wire in a vertical weld. With other wires, such as T-5 wires, an angle between zero and five degrees is recommended as it will impart more heat into the base material and allow the slag to rise to the surface of the weld pool.
  • Travel Speed: It is important to get your welding travel speeds correct to reduce the risk of slag inclusion. Travelling too fast can lead to a convex bead profile with dips at the weld toes that can lock slag in place.
  • Voltage: Too high a voltage can also cause the weld bead profile to become concave, trapping weld slag and leading to an inclusion.

How to get Rid of Welding Slag

Some types of welding slag are easier to remove than others, depending on what the flux was made from. Acid or rutile fluxes are free from fluoride, creating a slag that is easy to remove but more basic fluxes contain calcium fluoride, which makes them more difficult to remove.

Because slag doesn’t add to weld strength or protection after the welding process is complete, welders remove the slag to:

  1. Be able to clearly inspect the weld area
  2. For a more pleasing visual appearance
  3. To make subsequent weld passes
  4. To clean the surface so coatings can be applied, including paints or oil.

Slag removal is usually done with manual or power tools. Manual tools include chipping hammers, and wire brushes, while power tools include angle grinders with wire brush wheels or disks.

However, some filler materials are created to include self-peeling slag that will release itself from the weld without the need for chipping or grinding away.

Sometimes welding can create unsightly welding spatter on other nearby materials. Like the welding slag that covers the molten weld pool, this will also often need removing. Again, the same power and manual tools can be used in many instances but, in others, removal is more difficult, such as with glass…

How to get Welding Slag off Glass

When welding spatter (also known as ‘splatter’) contacts glass it can be a problem to remove, so obviously the key is to try and avoid it happening in the first place. However, if it does happen there are a number of things you can try to remove it.

  1. Scrub Immediately with a Brush: If you notice that welding slag or spatter is on the glass it is best to try and act as quickly as possible. Using a scrubbing brush it is possible to remove most of the spatter before oxidation. You may need to cover surrounding areas to prevent particles from scratching the glass while cleaning. Water pressure, such as from a hose, can help to dissolve any oils while scrubbing but you may need to also use a mild abrasive to clean any remaining residue.
  2. Soak with Acetone: Acetone or ethyl alcohol (known as rubbing alcohol) can remove weld spatter from glass. These products can break down oil and grease bonds but may take some time for the liquid to evaporate.
  3. Use a Commercial Solvent: You may need something stronger than acetone to work quicker and there are a number of commercial solvents and degreasers available that may work. Don’t use anything too strong, such as paint stripper or brake cleaner, as they can cause damage.
  4. Try a Wax and Oil Remover: A wax and oil remover can be used to cut through welding spatter on glass, just as it would be used for car paint removal jobs.
  5. Sand and Polish: If the above methods do not work, you may need to sand down the affected area to remove the spatter before polishing it back up using car polish and cutting compound or one of the available glass polishing products for the job. The spatter can also be removed with a razor blade ahead of polishing.


While welding slag can be messy, it does serve a purpose in protecting the weld zone and can even aid with vertical welding jobs.

However, it can be unsightly and make weld inspection difficult while also acting as a barrier against subsequent weld passes or the application of coatings.

Weld slag (and spatter) often needs to be cleaned away by grinding or brushing but this will prevent further problems later with the structural integrity of your weld. Weld slag inclusions are another problem that can arise from poor quality flux or welding technique. Inclusions can introduce defect to your weld and need to be ground or drilled out before the weld is attempted again.


What Type of Welding Creates Slag?

Slag is created by welding methods including electroslag welding, flux-cored arc welding (both self-shielded and gas-shielded), and submerged arc welding. The thickness of the slag that is produced depends upon the type of flux and the amount of flux in the electrode.

Can You Weld Over Slag?

It is possible to weld over slag, but the slag that is left underneath the weld puddle is likely to contaminate the new weld, especially if it is heavy slag. An experienced welder may be able to complete more than one pass without cleaning slag from a past weld bead, but generally this is not recommended. However, to achieve this the arc power should be sufficient to push slag on the weld puddle back from the joint. The more energy that is used, the more slag is produced, plus the arc may jump and create spatter.

What Happens if You Weld Over Slag?

When you weld over slag, you run the risk of introducing non-metallic inclusions from the flux directly into your weld. This will create weld defects that weaken the join. It is recommended to clean away welding slag between welding passes.

Is there Slag in MIG Welding?

While slag (and slag inclusions) are usually associated with flux-cored arc welding, stick welding and submerged arc welding, it is also possible to get inclusions from MIG welding.

What is the Purpose of Slag in Welding?

Welding slag can cause problems when welding, but it also serves a purpose. The slag is created by the melting flux coating during welding. Once it melts, the slag should rise through the weld pool and forms a shield across the top of the weld pool that protects the molten metal from contamination and oxidising atmospheric gases. As it is a non-metallic by-product, the slag needs to be removed t leave a clean metal join, especially if subsequent welding passes are to be made. Any slag that is left behind can weaken the next weld layer that is applied.

As well as containing the flux, the slag also contains atmospheric gases and impurities that are absorbed by the flux during melting.

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