Crust Has Blisters (Blown Crust) — Causes & Fixes

Small or large blisters on sourdough crust? Learn the common causes, quick fixes, and prevention strategies grounded in sourdough science.

Quick Diagnosis

WHAT are the blisters like and when do they appear?

Small, random blisters across the crust after baking Large single bubbles or popped blisters Blisters that form under the skin (separated thin crust) Blisters mainly on the bottom of the loaf

Causes & Solutions

Excess surface steam early in bake

common

Symptoms:

  • • Many small glossy blisters on top of crust
  • • Crust appears separated from interior in spots

Why does this happen?

High humidity in the first 10–15 minutes keeps crust soft; trapped expanding gas forms blisters beneath the surface as the crust sets later. Managing early steam changes blister formation [1].

🚨 Immediate Fix:

Next bake: reduce the initial steam source. If using a Dutch oven, remove the lid a few minutes earlier to let moisture escape, or reduce added water to steam tray.[1]

📅 Long-term Fix:

Use a steaming method you can control (Dutch oven or cloche) and avoid over-saturating the oven floor with water. Practice controlled venting after 10–15 minutes.[2]

🧪 Test:

Compare two bakes identically shaped — one with a closed [Dutch oven](https://amzn.to/4sVhKhN) full steam and one with limited steam to see blister differences.

Underproofed dough during final proof

common

Symptoms:

  • • Large blisters or bulges that pop during scoring or early oven spring
  • • Dense crumb pockets below the blister

Why does this happen?

Underproofed dough traps large gas pockets that rapidly expand in the oven; the still-tight gluten snaps and creates large blisters or tears rather than a uniform oven spring [1].

🚨 Immediate Fix:

If you catch it pre-bake: allow a short additional bench rest or longer final proof at a warmer spot (use a proofing box or near a warm appliance).[1]

📅 Long-term Fix:

Learn your dough's proofing indicators: jounce test and finger dent recovery. Slightly longer proof to allow gas redistribution.[2]

🧪 Test:

Poke test: a well-proofed loaf slowly springs back; underproofed dough springs back quickly and strongly.

Too-high hydration without enough strength

medium

Symptoms:

  • • Thin, papery crust with blister pockets
  • • Very open, irregular crumb beneath blisters

Why does this happen?

High hydration increases internal steam and gas mobility. If gluten hasn't developed enough to hold gas distribution evenly, large pockets migrate to the surface and blister the crust [2].

🚨 Immediate Fix:

Next time: build more gluten before final proof via extra folds or a short coil fold during bulk fermentation. Use a dough scraper to handle wet dough gently.

📅 Long-term Fix:

Progressively increase hydration with improved handling: stretch-and-fold schedule and stronger grist (higher-protein flour) to balance hydration.[1]

🧪 Test:

Make two loaves with the same hydration but different fold schedules; compare blistering and crumb structure.

Poor shaping leaves entrapped surface gas

common

Symptoms:

  • • Blisters located along seams or folds
  • • Blisters appear where dough was handled last

Why does this happen?

Trapped surface gas from sloppy shaping or insufficient degassing can expand under the skin during oven spring and form localized blisters [2].

🚨 Immediate Fix:

Before scoring, gently degas the surface with a light pat and tighten the seam during shaping using a bench knife or hands.

📅 Long-term Fix:

Refine shaping technique: build surface tension and seam lock so gas distributes internally—not at the skin.[1]

🧪 Test:

Reshape a second loaf with deliberate tighter surface and compare blister patterns.

Flour or dough dust on crust (creates insulating layer)

medium

Symptoms:

  • • Paper-thin film or flour patches with blisters underneath
  • • Blisters follow areas with excessive dusting

Why does this happen?

Excess surface flour or wet dust (water-heavy dusting) can form a thin barrier that sets into a fragile skin; steam/gas then separates it, producing blisters [2].

🚨 Immediate Fix:

Brush off excess flour before final proof and avoid wetting the surface. Use a light dusting only; if you used a banneton, line it carefully with linen.

📅 Long-term Fix:

Adopt minimal dusting and practice releasing dough without over-flouring. Use rice flour sparingly for sticking spots only [1].

🧪 Test:

Bake a loaf with normal vs excessive dusting to observe blister frequency.

Excess reactive enzymes from fresh whole-grain flour

rare

Symptoms:

  • • Crust separates in patches, sometimes shiny blisters
  • • Loaf made with large percentage of fresh whole-grain flour

Why does this happen?

Fresher whole-grain flours have active enzymes that increase gas production and change crust setting behavior; combined with steam this can create blisters in some formulas [2].

🚨 Immediate Fix:

Reduce whole-grain percentage or rest whole-grain preferment before using. Slightly longer bulk can mellow enzymatic action.

📅 Long-term Fix:

Store whole-grain flours cool and experiment with lower fresh-grain percentages in high-hydration bakes [2].

🧪 Test:

Substitute part of whole-grain flour with white flour and compare blisters.

Scoring or lame technique traps gas

common

Symptoms:

  • • Blisters adjacent to or under scores
  • • Scores that don't open cleanly

Why does this happen?

Too-shallow or irregular scoring can fail to direct oven expansion; trapped gas finds weak spots and causes blisters rather than clean ear formation [1].

🚨 Immediate Fix:

Score with a sharp bread lame/knife at a shallow angle and ensure a single confident stroke.

📅 Long-term Fix:

Practice consistent scoring depth and angle; for high-hydration loaves keep slashes slightly deeper to give expansion a path.[1]

🧪 Test:

Experiment scoring angles — the same dough scored shallow vs deeper will show different blister outcomes.

🆘 Can I fix blisters now?

Large blister popped immediately after scoring or during bake

Solution: If you see it in the oven, quickly open the oven and vent steam to encourage crust setting. After baking, cool fully; cut out any excessively thin blistered parts before serving.

Success chance: moderate [1][2]

Multiple small blisters on finished loaf

Solution: No structural fix post-bake. Accept as cosmetic; crisp blistered crust can be pleasant. For sandwiches, trim uneven crust. For future loaves, apply the prevention checklist below.

Success chance: good for taste, cosmetic only [2]

Underproof suspected and blistering severe

Solution: For next time: proof a test boule longer at warmer temp or give a short cold retard in the fridge to even gas distribution. You cannot reliably fix an already-baked loaf.

Success chance: prevents recurrence [1]

Prevention

  • ☐ Control initial oven steam: use a Dutch oven or cloche and vent after 10–15 min
  • ☐ Improve gluten strength with extra folds during bulk using a dough scraper
  • ☐ Judge proofing by feel (poke/jounce) not only time; avoid underproofing
  • ☐ Minimise surface dusting; line your banneton properly
  • ☐ Score confidently with a sharp bread lame/knife to give gas an exit path
  • ☐ For high-hydration loaves, build strength with longer bulk or stronger flour before increasing hydration

Sources

  1. [1]
    The Perfect Loaf – The Perfect Loaf – Link
  2. [2]
    Plötzblog – Plötzblog – Link