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Weld spatter: what it is and how to spot it

Fillet Gauge editorial teamSources checked 3 min read

Spatter is small droplets of molten metal thrown out of the arc during welding that land and stick on the base metal or the weld beside the joint. The droplets freeze as raised beads, from fine specks to larger blobs. Spatter is a surface condition, not a flaw inside the weld, but it can hide other problems and spoil coatings.

A weld bead seen from above with small raised droplets of metal stuck to the plate on both sides of the bead
Raised dots on the plate, not holes in the weld. Original drawing, not to scale.
Use this diagram

How to recognize spatter

  1. Feel or light it from the side. Spatter stands up from the surface. In low-angle light each droplet casts a small shadow on the far side.
  2. Look either side of the weld. It lands on the plate along both sides of the bead, thickest near the weld and thinning out with distance.
  3. Check how well it is stuck. Loose spatter flicks off with a chipping hammer or scraper. Spatter that fused hard into the plate leaves a small mark when removed.
  4. Look at what it covers. Heavy spatter can hide the toe, undercut or a crack under it. Remove it before you judge what is underneath.

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Welding Inspection and Discontinuities

A convex fillet weld replica is checked with a leaf-type fillet gauge. The drawing calls for a single fillet size. The vertical leg fits the 3/8 in blade and the horizontal leg the 5/16 in blade. What size is recorded?

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How it is judged

Spatter is found visually. Whether it is acceptable depends on the job: many documents leave it to the contract, the coating system or the engineer, and a part going to be painted or galvanized often needs it removed because coatings won't cover it well. When inspecting, note its extent and whether it blocks your view of the weld. Look up what the governing document or contract says rather than assuming it never matters.

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Common mix-ups

  • Spatter or porosity. Spatter is a raised dot on the surface. Porosity is a hole in the weld metal. One sticks up, the other goes in.
  • Spatter or an arc strike. An arc strike is a spot where the base metal melted. Spatter is metal that landed on the plate from the arc.
  • Spatter or slag. Slag is the glassy coating from the flux, on top of the bead. Spatter is metal, and it lands beside the weld.

Why it forms

The arc and the transfer of metal across it are unstable. Common causes are too high a current or voltage for the process, a long arc, a steep electrode angle, damp or contaminated consumables, magnetic arc blow, and in GMAW the transfer mode: short-circuit and globular transfer throw more spatter than spray or pulsed transfer. Shielding gas choice matters too, with straight carbon dioxide usually giving more spatter than argon mixes.

Spatter on the CWI exam

On Part A, spatter belongs to Visual Examination and Joining and Cutting Processes, at least 14% of the exam, where transfer modes and parameter effects are covered. On Part B it fits Welding Inspection and Discontinuities, at least 34% of the practical, mainly as a look-alike to separate from porosity and arc strikes on a replica. On Part C, check how your code treats it in the workmanship and visual inspection clauses of your own copy.

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Separate spatter from porosity and arc strikes in the weld discontinuities quiz. Then read the two it gets confused with: arc strikes and weld porosity. Every type is in the weld discontinuities guide.

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Sources

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Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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