Invisible Salt, Visible Damage
An introduction to salt weathering and phase diagrams for art students — featuring the IoT monitoring of Huang Tu-shui's Water Buffaloes

Let's start with a story.
You bring home a beautiful stone from the beach and put it on your desk. Months later its surface starts to powder, showing white dust and even flaking off — yet nobody dropped or hit it. Who is the culprit?

The culprit is salt hiding inside the stone's capillary pores. Moisture in the air makes the salt repeatedly absorb water, dissolve, dry out, and re-crystallize. Each cycle acts like a tiny jack pushing the stone apart from the inside. This is salt weathering — one of the most common causes of deterioration for stone sculpture, murals, brick walls, and ceramics worldwide.

This lesson teaches you to read a "weather map for salt" (a phase diagram), so you can interpret museum temperature–humidity records and know when an artifact is safe and when it is being damaged.
Course Map
  1. What is humidity? — Air is a sponge
  2. Why does salt absorb water? — Deliquescence
  3. The most dangerous moment: re-crystallization — Crystallization pressure
  4. Meet the protagonist: the "shape-shifting" of sodium sulfate (Na₂SO₄)
  5. The phase diagram = a map for salt
  6. Hands-on: drag T & RH and watch the salt
  7. Real case: a full year of the Water Buffaloes
  8. Quiz and glossary

1. What is humidity? — Air is a sponge

What does "80% humidity today" actually mean?

Think of air as a sponge, and water vapor as the water it holds.
Relative humidity (RH) = how full the sponge currently is.
RH 50% = half full; RH 100% = completely full — any more gets "squeezed out" as dew or fog.

The key point: the warmer the air, the bigger the sponge. The same amount of water vapor fills only a small part of the sponge on a hot day (low RH), but at night the sponge shrinks and quickly approaches full (high RH). That is why RH peaks around midnight and dawn and drops in the afternoon — even though the actual amount of vapor never changed.
Same water vapor, different temperature → completely different RH ☀️ Hot day 30°C Air sponge (large) ⛅ Mild 22°C Sponge shrinks 🌙 Cold night 14°C Even smaller RH ≈ 45% (still empty) RH ≈ 70% RH ≈ 95% (nearly full!) The number of droplets (vapor amount) is identical in all three panels — only the "sponge" (air capacity) changes
Fig. 1 | Why RH soars after evening cool-down: vapor didn't increase — the air's "capacity" shrank.
Relative humidity (RH) tells you "how far the air is from saturation", in %. It changes constantly with temperature, so conservation monitoring must record temperature and humidity together — exactly what the IoT sensors do every 10 minutes.

2. Why does salt absorb water? — Deliquescence

You have surely seen it: the salt shaker clumps in the rainy season, and unsealed cookies go soft. Both grab water from the air.
Even more striking: once humidity exceeds a certain threshold, salt grabs so much water that it dissolves itself completely into a droplet of brine — this is called deliquescence.
Deliquescence: what happens to salt as RH keeps rising? ① RH below threshold Dry crystals, well-behaved ② RH reaches threshold Starts grabbing water; a film forms ③ RH above threshold Brine Absorbs until fully dissolved = deliquescence In reverse (RH falling) it walks from ③ back to ① — and on that way the salt re-crystallizes, the step that hurts artifacts most
Fig. 2 | Deliquescence in three acts. NaCl's threshold is ~75% RH; every salt differs.

Every salt has its own deliquescence threshold. For familiar table salt (NaCl) it is about RH 75%:

Ambient RHState of table salt
Below 75%Dry solid crystals
Above 75%Absorbs water and dissolves into brine

This is why the advanced tutorial's diagram has an orange horizontal line (75%) — the deliquescence line of NaCl.

Deliquescence RH ranking of common salts at 20 °C A vertical humidity scale comparing the thresholds of MgCl2, NaCl, mirabilite and K2SO4, with the 60-85% RH range common in Taiwanese museums. The threshold is a "water switch": it flips on only when RH reaches the line At 20°C; the lower the threshold, the more easily it absorbs and dissolves Typical museum RH in Taiwan 60–85% Salts whose threshold falls inside this range may already be "on" Watch the threshold, not just the name 20% 40% 60% 80% 100% MgCl₂　33% Magnesium chloride: absorbs even indoors NaCl　75% Table salt: everyday RH can cross it Na₂SO₄·10H₂O　95.6% Mirabilite: deliquesces only when very humid K₂SO₄　97% Potassium sulfate RH rises → crosses the dot → water switch turns on Note: Na₂SO₄ also shape-shifts (anhydrous ⇌ decahydrate) before it ever deliquesces
Fig. B | Thresholds vary greatly between salts; each one's water switch opens only when ambient RH crosses its own line. (Designed by GPT-5.6 Sol / Codex)
The deliquescence threshold = the equilibrium RH of that salt's saturated solution. When ambient RH oscillates across the threshold, the salt cycles between dissolving and crystallizing.

3. The most dangerous moment: re-crystallization — Crystallization pressure

Put a water-filled glass bottle in the freezer and it cracks overnight — ice expands and bursts the bottle.
Salt does the same: brine hides in the stone's capillary pores, and when RH drops and water evaporates, the salt re-grows crystals inside the pores. Growing crystals need space, so they push the pore walls outward. This force — crystallization pressure — can exceed the strength of the stone itself.
A single crystallization won't destroy an artifact; the danger is the repeated cycle: dry by day (crystallize) → humid at night (dissolve) → dry again (re-crystallize)… Hundreds of cycles a year are like hundreds of micro-blasts, and the surface starts powdering, scaling, and flaking.
Crystallization pressure: a micro-jack inside the stone Stone cross-section Capillary pore Crystal grows 1️⃣ Brine seeps into the pore 2️⃣ RH drops, water evaporates 3️⃣ Salt re-crystallizes and grows in the pore 4️⃣ Crystallization pressure cracks the pore wall 5️⃣ Hundreds of cycles → powdering, flaking Like ice bursting a bottle in the freezer — too small to see, too frequent to resist
Fig. 3 | The direct mechanism of salt damage: pressure in capillary pores can exceed stone strength.

4. Meet the protagonist: the "shape-shifting" of Na₂SO₄

Our protagonist is sodium sulfate. It is called "the most aggressive salt for stone" because it has one more trick than table salt — shape-shifting. It exists in two forms:

Thenardite (anhydrous)Mirabilite (decahydrate)
FormulaNa₂SO₄ (no water)Na₂SO₄·10H₂O (10 water molecules)
Looks likeDry powderWater-saturated crystals
Appears inDry conditionsHumid conditions
Thenardite turning into mirabilite is like a dried mushroom soaking back into a plump one — except this "rehydration" expands the volume more than threefold, inside capillary pores with no room to stretch.
The shape-shift: a cycle that swells 3× on hydration Volume 1 Thenardite Na₂SO₄ (dried mushroom) Volume ×3.1 Mirabilite Na₂SO₄·10H₂O (soaked mushroom) RH rises past the transition line: grabs 10 H₂O → swells! RH falls below the line: releases H₂O → shrinks, loosens Repeated swelling/shrinking in pores = repeated micro-blasts (289 times in one year at the Water Buffaloes gallery!)
Fig. 4 | The thenardite ⇌ mirabilite hydration cycle — why Na₂SO₄ is fiercer than NaCl.
So sodium sulfate has two damage modes:
① the same dissolve⇌crystallize cycle as table salt;
② its signature anhydrous⇌decahydrate shape-shift, each accompanied by a huge volume change.
Whenever ambient T–RH oscillates near the threshold, both cycles play out daily.

5. The phase diagram = a map for salt

In 2008, Steiger and Asmussen precisely computed which form sodium sulfate "should" take at every temperature and humidity, and drew it as a phase diagram. A "phase" simply means a form or state.

A phase diagram is really a map: temperature on the x-axis, humidity on the y-axis; every point is a kind of "weather". Two blue lines divide the map into three countries:
🔴 Solution Land (upper-right, humid): all salt dissolved into brine.
🟢 Mirabilite Land (middle): water-rich mirabilite is stable.
🟠 Thenardite Land (lower, dry): dry thenardite is stable.

The two blue lines in plain words:

LinePlain nameWhat happens when crossed
Upper blue line (deliquescence)The "dissolving line"Crossing up: salt dissolves (deceptively calm).
Crossing down: brine bursts into crystals → crystallization pressure strikes. Most dangerous!
Lower blue line (hydration)The "transition line"Crossing up: anhydrous→decahydrate, volume swells 3×.
Crossing down: decahydrate→anhydrous, shrinks and loosens. Repeated crossings = repeated blasts.

The two lines merge at about 32.4 °C (above that, mirabilite no longer forms). If you remember one thing: the closer the data sit to a line and the more often they cross it, the greater the danger.

One "blast cycle" can complete within a single day Time of day Relative humidity MidnightDawnNoonAfternoonLate night Transition line (thenardite ⇌ mirabilite) Humid night → mirabilite (swollen state) Dry daytime → dehydrates to thenardite ⚡ Morning: crosses down, dehydrates ⚡ Night: crosses up, hydrates Each ⚡ is a phase change. The natural day–night rhythm alone makes the salt shape-shift daily.
Fig. 5 | Why the data cross the line ~300 times a year: the daily cycle itself repeatedly crosses the transition line.
A day is not a point — it walks a loop on the T–RH map ① Dawn, most humid → ② warming & drying: ⚡crosses the line = dehydration (mirabilite → thenardite) ③ Afternoon, driest → ④ cooling & re-humidifying: ⚡crosses back = hydration (swells 3×) → back to ① 10203040 40557085100 Temperature (°C) → Relative humidity RH (%) → Solution zone (all dissolved) Mirabilite zone (10 H₂O) Thenardite zone (dry) Deliquescence line Transition line Triple point 32.38°C / 87.42% ⚡⚡ ① Dawn | most humid ② ⚡Dehydration ③ Afternoon | hottest & driest ④ ⚡Hydration Red loop = one day's journey; each touch of the dashed line (⚡) is one shape-shift — nearly 300 per year
Fig. C | One day's T–RH drawn on the phase diagram: where the closed loop meets the blue line is when the salt shape-shifts. (Designed by GPT-5.6 Sol / Codex)

6. Hands-on: drag T & RH and watch the salt

Drag the two sliders to simulate gallery conditions and see which "country" the salt is in:

7. Real case: a full year of the Water Buffaloes

The gallery of Huang Tu-shui's Water Buffaloes has two sensor sets. From 2021-08-09 to 2022-09-22 they recorded 53,779 T–RH readings (about every 10 minutes). Plotting every reading on the phase diagram:

7.1 Ambient gallery air — trouble

Ambient phase diagram
Green = Mirabilite Land, orange = Thenardite Land, red = Solution Land. The data cloud straddles the transition line, with a few points bursting into Solution Land.
Findings:
① In one year the data crossed the transition line 289 times (145 dehydrations + 144 hydrations) — if gallery walls or stone contain Na₂SO₄, that is nearly 300 micro-blasts a year.
② The dissolving line was crossed 14 times, concentrated on 2022-04-24, 2022-07-04–05, 2022-07-10–12 (days when RH hit 93–94%). Seven were solution→crystal events — the exact moments crystallization pressure struck.
Timeline
The same data laid out in time: color = current phase zone. The year-long bouncing between green (decahydrate) and orange (anhydrous) is obvious.

7.2 The showcase microclimate — safe

Microclimate phase diagram
All 53,779 readings inside the showcase stay in Thenardite Land (RH max 66.9%) — not a single crossing.
How a showcase protects salt-sensitive works Side-by-side: gallery air vs showcase microclimate. Gallery T-RH swings across the salt transition line many times; inside the case RH is steady with zero crossings all year. The showcase is a "buffer sponge": it flattens the big outside waves Gallery air Doors, crowds and weather all push T–RH around Salt transition line Crossed the transition line 289 times/yr incl. 14 deliquescence-line crossings Showcase damps swings Microclimate inside the showcase Salt transition line 0 crossings all year | RH max 66.9% The goal is not a perfectly constant climate — it is never crossing the danger thresholds
Fig. A | The showcase shrinks the gallery's big swings so the microclimate never crosses the salt-damage thresholds. (Designed by GPT-5.6 Sol / Codex)
This is the value of microclimate control: wrap the artifact in a small climate that never crosses the lines. Even if salt is present, it stays locked in one form and cannot act up. The showcase protects the sculpture itself; what needs attention is the building and materials exposed to the ambient air.

8. Quiz

Q1. Why is RH usually higher at midnight than in the afternoon?
Cooling shrinks the "air sponge" — the same vapor fills a larger fraction, so RH rises.
Q2. Which moment is more dangerous for an artifact: RH surging to 95% (dissolving the salt), or RH falling from 95% back to 90%?
The fall — brine re-crystallizes and crystallization pressure erupts in the pores. Dissolution itself is quiet.
Q3. A gallery sits at 25 °C with RH oscillating between 79% and 83% all day. Check with the widget in Section 6: good news or bad news for stone containing Na₂SO₄?
Bad news! At 25 °C the transition line sits at ~80.7%; oscillating across it all day means constant anhydrous⇌decahydrate shifting — the most damaging situation.

9. Glossary

Relative humidity (RH)
How saturated the air is with vapor, 0–100%. Varies with temperature.
Deliquescence
Above its threshold RH, a salt absorbs water until it dissolves itself into brine.
Crystallization pressure
The force a growing crystal exerts on pore walls; the direct cause of salt damage.
Thenardite
Na₂SO₄, the dry form without crystal water.
Mirabilite
Na₂SO₄·10H₂O, the ten-water form produced when thenardite hydrates, more than 3× larger in volume.
Phase / phase diagram
A phase is a form of matter; a phase diagram is the map of which form is stable at each T–RH.
Phase boundary (blue lines)
The border between two phase zones. Crossing it = a phase change (dissolving, crystallizing, hydrating, dehydrating).
Microclimate
The small climate right around an artifact (inside a showcase or frame), which can differ completely from the gallery.

10. Want to go deeper?

After this lesson, try the advanced tutorial the advanced tutorial (English): the full mathematics of Steiger's theory, the Python and R code, and the complete list of all 303 crossing events. You can also open the interactive phase diagram interactive_phase_peripheral.html — hover over any data point to see its exact date and time.

🌐 中文