Rochelle Salt — Crystal Growing and Piezoelectric Microphone

Grow large crystals of sodium potassium tartrate from kitchen chemicals and build a working crystal microphone
Advanced🕐60 min●Low hazardcrystallizationphysical-properties

Summary

You’ll make sodium potassium tartrate from cream of tartar and baking soda, grow a large single crystal1, and use its piezoelectric2 effect to build a working microphone. You’ll be able to explain why a seeded crystal grows, why an asymmetric crystal makes voltage from pressure, and why the effect peaks near room temperature.

History

Rochelle salt (sodium potassium tartrate tetrahydrate, NaKC₄H₄O₆·4H₂O) was first crystallised around 1675 by Élie Seignette3, an apothecary in La Rochelle, France, who sold it as a laxative under the name Seignette salt. For two centuries it remained a medical curiosity.

In 1880 Pierre and Jacques Curie, experimenting with crystals that lacked a centre of symmetry, discovered that applying mechanical pressure generated a measurable electric charge. They called the effect piezoelectricity (from the Greek piezein, to press). Rochelle salt was one of their test materials, and it turned out to have by far the strongest piezoelectric response of any crystal then known — several hundred times larger than quartz.

The commercial applications followed quickly. From the 1920s through the 1960s, Rochelle salt was the active element in crystal microphones, crystal phonograph cartridges, and telephone receivers. It has since been replaced in most applications by more robust synthetic piezoelectric ceramics (PZT), but it remains the only piezoelectric material you can grow yourself in an afternoon from kitchen ingredients.

Hazards & preparation

Warning

PPE: none required — Rochelle salt, cream of tartar, and baking soda are all food-grade and non-toxic. This is one of the safest experiments here.

  • Take care with the hot solution when dissolving (it’s ~70 °C).
  • Handle epoxy (for the microphone) with gloves — it’s a mild irritant uncured.
  • If you use an amplifier, follow normal mains-electrical safety.

Disposal: the spent solution is dilute sodium potassium tartrate — pour it down the drain. See the Safety page.

Part 1 — Synthesising Rochelle Salt

Rochelle salt is made by reacting cream of tartar (potassium hydrogen tartrate) with baking soda:

\[\ce{KHC4H4O6 + NaHCO3 -> NaKC4H4O6 + H2O + CO2 ^}\]

Both starting materials are cheap food-grade chemicals.

Materials

  • Cream of tartar (potassium hydrogen tartrate) — 75 g
  • Baking soda (NaHCO₃) — 28 g (exactly 1:1 molar ratio: MW of KHC₄H₄O₆ = 188, MW of NaHCO₃ = 84)
  • Distilled or filtered water — 200 mL
  • Small saucepan
  • Coffee filter or fine cloth for filtering
  • Clean glass jar (500 mL) with lid

Procedure

  1. Heat 200 mL water to about 70°C (hot but not boiling).
  2. Add the cream of tartar and stir until dissolved (it dissolves slowly; warming helps).
  3. Add the baking soda a little at a time, stirring between additions — it fizzes vigorously as CO₂ is released. Add slowly to avoid overflow.
  4. Once fizzing stops, check the pH is nearly neutral (6.5–7.5). If still acidic, add a pinch more baking soda.
  5. Filter the hot solution through a coffee filter into the clean jar.
  6. Cool to room temperature — fine white Rochelle-salt crystals form. Save a few as seed crystals.

The yield is about 90–100 g.

Part 2 — Growing Large Crystals

A piezoelectric microphone needs a crystal at least 1–2 cm across, which requires slow recrystallisation onto a single seed.

  1. Redissolve the Rochelle salt in 150 mL water at 60°C.
  2. Cool to 35°C, then remove the thread-tied seed and set aside (so the solution is saturated at a slightly elevated temperature).
  3. Let it cool to ~20°C undisturbed for 15–20 minutes; remove any fine spontaneous crystals.
  4. Suspend the seed crystal (tied to a thread hung from a pencil) so it floats freely, not touching the sides.
  5. Cover loosely and keep in a cool, stable, vibration-free spot (18–22°C).
  6. Check daily; remove any “parasitic” crystals on the thread or walls that compete with the seed.

Growth rate: 1–3 mm/day. A usable crystal (~15 × 10 × 8 mm) takes 4–7 days. Temperature stability matters most — keep daily swings under 3°C, or the crystal alternately dissolves and regrows and turns cloudy. Store finished crystals sealed (they’re hygroscopic) or coat with clear nail varnish.

Part 3 — The Piezoelectric Microphone

The bimorph bender

Rochelle salt microphones use a bimorph: two crystal slabs cemented with their piezoelectric axes opposed, so that when the assembly bends, one slab is compressed and the other stretched and their voltages add. A simplified version uses a single slab glued to a thin flexible backing. A metal-foil diaphragm converts sound pressure into bending.

Materials

  • Rochelle salt crystal, at least 15 × 10 mm face, 5+ mm thick
  • Aluminium foil (for electrodes)
  • Conductive paint or graphite pencil (optional, to improve contact)
  • Thin flexible plastic sheet or card
  • Two-component epoxy glue
  • Two thin insulated wires, stripped at the ends; alligator clips
  • A high-impedance amplifier input (guitar amp instrument input, audio interface, or an op-amp buffer)
  • Multimeter capable of mV readings

Assembly

  1. Cleave a slab: press a sharp blade gently along a natural cleavage plane to split off a flat piece 15 × 10 mm and 3–6 mm thick.
  2. Prepare electrodes: cut two foil pieces slightly smaller than the crystal faces (optionally rub graphite on the faces for better contact).
  3. Sandwich: foil–crystal–foil, pressed flat so the foil makes full contact (air gaps kill sensitivity).
  4. Attach wires: fold a tab out of each foil and attach a wire (conductive epoxy, clip, or wrap-and-tape). Polarity is arbitrary.
  5. Optional backing: glue the sandwich to a thin flexible sheet at one end only, leaving most free to flex.
  6. Protect: wrap loosely in cling film to keep moisture out, wires poking out.

Testing

With a multimeter (AC millivolts): tap the crystal — you should see brief pulses of a few millivolts, confirming piezoelectric activity.

As a microphone: connect to a high-impedance input (≥500 kΩ, ideally 1 MΩ+) — a guitar amp instrument input or audio interface. Speak or tap near it; the output is audible/visible. Quality is poor and the response uneven, but the signal is unmistakable.

Note

Improving sensitivity: an op-amp voltage follower (e.g. a TL071 wired as a unity-gain buffer, powered from a 9V battery) between the crystal and the audio input prevents impedance loading and can triple the usable output.

What you should see

Part 1 clouds and fizzes, then drops a snow of white crystals as it cools. Over the next week the seed swells into a clear, glassy, chisel-shaped single crystal you can hold up to the light. And when you tap the finished crystal-and-foil sandwich, the multimeter flicks with millivolt pulses — pressure turned directly into electricity.

Symptom Likely cause Fix
Crystal grows cloudy/rough Growth too fast, or temperature swings Stabilise temperature (<3°C/day); warm briefly to dissolve the rough layer, then resume
Crystal stops growing Solution no longer supersaturated Dissolve a little more Rochelle salt in minimal hot water and add carefully
Parasitic crystals everywhere Dust or seeds in solution Filter; remove stray crystals promptly
No voltage from the mic Air gap / low-impedance input Press foil into full contact; use a ≥1 MΩ input or an op-amp buffer

The Science

Why Rochelle salt is piezoelectric

Piezoelectricity requires a crystal with no centre of inversion symmetry — where positive and negative charge centres are offset. Deforming such a crystal shifts the charge distribution and produces a voltage between opposing faces. Rochelle salt is ferroelectric between −18°C and +24°C, with a spontaneous polarisation switchable by a field (like magnetic domains in a ferromagnet). Near its upper Curie temperature (~24°C) the piezoelectric coefficient rises dramatically — which is why its response near room temperature is so large, far exceeding quartz. Above ~25°C the polarisation vanishes and the response drops sharply, so a crystal mic works best in a cool room.

The piezoelectric relation

\[\sigma = d \cdot \varepsilon\]

where \(\sigma\) is surface charge density, \(d\) the piezoelectric coefficient, and \(\varepsilon\) the applied stress. Rochelle salt’s \(d\) is ~170–350 pC/N — roughly 100× quartz’s ~2–3 pC/N. That huge value comes from sitting so near its Curie transition, where susceptibilities peak; quartz has no nearby transition, so it’s weaker but rock-stable with temperature.

Questions to Explore

  1. What makes a crystal grow at all? Why do molecules add to the seed rather than form new tiny crystals, and why does temperature stability matter?

    Hint / answer

    Building onto an existing surface is energetically easier than starting a brand-new crystal from scratch, so in a gently supersaturated solution molecules join the seed. Stable temperature keeps the solution just-supersaturated; swings make it alternately dissolve and dump crystals, giving cloudy, flawed growth.

  2. Why must the crystal be asymmetric to be piezoelectric? What happens in a centrosymmetric crystal like table salt?

    Hint / answer

    In an asymmetric crystal, squeezing shifts the positive and negative charge centres by different amounts, creating a net voltage. In a centrosymmetric crystal every displacement is mirrored by an opposite one, so the charges’ shifts cancel and no voltage appears — symmetry forbids the effect.

  3. Why does the response peak near 24°C? Why does being near a phase transition boost the effect (and why the same in ferromagnets)?

    Hint / answer

    At the Curie point the crystal sits on the knife-edge between its polarised and non-polar phases, so its polarisation is extremely easy to nudge — a tiny stress produces a big charge shift. Ferromagnets behave the same near their Curie temperature: their magnetisation becomes very easy to change.

  4. Why did modern electronics replace Rochelle salt? What limitations, and what does PZT offer?

    Hint / answer

    Rochelle salt is temperature-sensitive (works only in a narrow cool band), hygroscopic, and mechanically fragile. PZT ceramics keep a strong, stable piezoelectric response from −200 to +300 °C and are tough and moisture-resistant, so they took over.

  5. What else uses piezoelectricity today? Watches, ultrasound, stove igniters, sonar — voltage-from-pressure or pressure-from-voltage?

    Hint / answer

    Stove igniters and microphones use pressure→voltage; ultrasound transducers and sonar run both ways (voltage→pressure to emit, pressure→voltage to receive); quartz watches apply voltage to make a crystal vibrate at a precise frequency. Quartz is chosen for timekeeping because its frequency is extremely stable with temperature.

Going further

  • Grow it in colour. Add a little food colouring to the growth solution to tint the crystal without spoiling its structure.
  • Make a phono pickup or knock sensor. Rest a blunt stylus on the crystal and drag it along a record groove, or mount it to detect taps and vibration.
  • Completes the Crystals & Solutions track: you’ve now grown crystals from precipitation, supersaturation, sublimation, and slow recrystallisation.

Footnotes

  1. Nucleation — The first formation of a tiny stable seed from which a crystal or bubble grows.↩︎

  2. Piezoelectric — Generating an electric voltage in response to mechanical stress.↩︎

  3. Pierre Seignette — French apothecary who first prepared Rochelle salt (Seignette salt) in the 1670s.↩︎