Rochelle Salt — Crystal Growing and Piezoelectric Microphone

Grow large crystals of sodium potassium tartrate from kitchen chemicals and build a working crystal microphone

Difficulty: Medium–Hard | Time: 3–7 days (crystal growth) + 1 hour (microphone) | Visual Impact: High

Historical Context

Rochelle salt (sodium potassium tartrate tetrahydrate, NaKC₄H₄O₆·4H₂O) was first crystallised around 1675 by Élie Seignette, 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. Nearly every household owned a device that depended on it. Rochelle salt 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.

Part 1 — Synthesising Rochelle Salt

Rochelle salt is made by reacting cream of tartar (potassium hydrogen tartrate, KHC₄H₄O₆) with baking soda (sodium bicarbonate, NaHCO₃):

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

Both starting materials are cheap food-grade chemicals available in any supermarket.

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 of water in the saucepan 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 will fizz vigorously as CO₂ is released. Add slowly to avoid overflow
  4. Once fizzing stops completely, verify the reaction is complete: a small amount of indicator paper should show a nearly neutral pH (6.5–7.5). If still strongly acidic, add a pinch more baking soda
  5. Filter the hot solution through a coffee filter directly into the clean glass jar to remove any undissolved particles
  6. Allow to cool to room temperature. Fine white crystals of Rochelle salt will begin to form as it cools

The yield is about 90–100 g of Rochelle salt. Save a few of these crystals as seed crystals before proceeding.

Part 2 — Growing Large Crystals

Small crystals form quickly on cooling, but a piezoelectric microphone needs a crystal at least 1–2 cm across. This requires slow recrystallisation — redissolving the crystals and allowing them to grow onto a single seed.

Materials

  • Rochelle salt from Part 1 (or all of it redissolved)
  • Seed crystal saved from first crystallisation
  • Thread or thin nylon line
  • Clean jar with lid or cling-film cover
  • Thermometer

Seed Crystal Preparation

Select the largest, clearest crystal from the first batch. Tie a short length of thread around it (loop through a natural groove if possible, or loop around it loosely). Suspend the seed from a pencil resting across the mouth of the jar so it hangs freely in the middle of the solution without touching the sides or bottom.

Growing Procedure

  1. Redissolve all the Rochelle salt (including the thread-tied seed) in 150 mL of water at 60°C
  2. Allow the solution to cool to 35°C, then quickly remove the seed crystal and set aside. This ensures the solution is saturated at a slightly elevated temperature
  3. Allow the solution to cool to room temperature (~20°C) undisturbed for 15–20 minutes. Some fine spontaneous crystals may form; carefully remove these without disturbing the solution
  4. Suspend the seed crystal in the solution
  5. Cover the jar loosely (to allow slow evaporation) and place in a cool, stable location (18–22°C) away from vibration
  6. Check daily. As the solution cools and slowly evaporates, supersaturation drives crystal growth onto the seed. Remove any “parasitic” crystals that form on the thread or jar walls as soon as you notice them — they compete with your seed

Growth rate: 1–3 mm per day under good conditions. A usable crystal (about 15 × 10 × 8 mm) typically takes 4–7 days.

Tips for Success

  • Temperature stability is the most important factor. Wide temperature swings cause the crystal to alternately dissolve and regrow, making it cloudy. Aim for a spot with less than 3°C of daily variation — a basement or inside a cabinet works well.
  • The optimal growth temperature is 18–22°C. Rochelle salt is most stable and grows most evenly here, and conveniently this is also the temperature range where its piezoelectric properties are strongest.
  • If the crystal stops growing, the solution is no longer supersaturated. Dissolve a small additional amount of Rochelle salt in minimal hot water, let cool slightly, and add carefully to the jar.
  • If the crystal becomes rough or milky rather than clear, the growth rate is too fast (too much supersaturation). Remove the crystal briefly, warm the solution slightly to dissolve the rough layer, then cool and resume.
  • Rochelle salt is hygroscopic above ~45°C and will decohere if heated. Store finished crystals in a sealed container, or coat with a thin layer of clear nail varnish to protect from humidity.

Part 3 — The Piezoelectric Microphone

Why Rochelle Salt Is Piezoelectric

Piezoelectricity requires a crystal with no centre of inversion symmetry — a crystal where the positive and negative charge centres are offset, but where opposite faces are not mirror images. When such a crystal is mechanically deformed, the charge displacement changes, and a net electric voltage appears between opposing faces.

Rochelle salt belongs to the orthorhombic crystal system and is ferroelectric between −18°C and +24°C: below +24°C it has a spontaneous electric polarisation that can be switched by an external field, analogous to the magnetic domains in a ferromagnet. Near this upper Curie temperature (~24°C), the piezoelectric coefficient rises dramatically — which is why Rochelle salt’s piezoelectric response is so exceptionally large near room temperature, far exceeding quartz.

Above ~25°C the ferroelectric polarisation vanishes and the piezoelectric response drops sharply. A crystal microphone therefore works best in a cool room (18–22°C), and poorly in a warm one (above ~28°C). This temperature sensitivity was the main reason Rochelle salt was eventually replaced by synthetic PZT ceramics, which work from −200°C to +300°C.

The Bimorph Bender

Rochelle salt microphones don’t simply compress a crystal — they use a bimorph: two crystal slabs cemented together with their piezoelectric axes opposed. When the assembly bends, one slab is compressed and the other stretched; both generate voltages that add together. A thin diaphragm (metal foil) attached to one end converts sound pressure into bending motion.

A simplified version omits the second slab and uses a single crystal slab glued to a thin flexible backing.

Materials

  • Rochelle salt crystal, at least 15 × 10 mm face, 5+ mm thick
  • Aluminium foil (for electrodes)
  • Conductive paint or graphite pencil (to improve contact) — optional
  • Thin flexible plastic sheet (from a food container) or thin card
  • Two-component epoxy glue
  • Two thin insulated wires, stripped at the ends
  • Alligator clips
  • High-impedance amplifier input: a guitar amplifier (instrument input), audio interface, or a simple op-amp buffer circuit
  • Multimeter capable of mV readings (to test without an amplifier)

Assembly

  1. Cleave or file a slab from the crystal. Rochelle salt has natural cleavage planes roughly parallel to its large faces. Press a clean sharp blade gently along a cleavage line and the crystal will split. Aim for a flat piece 15 × 10 mm or larger and 3–6 mm thick. It does not need to be perfect — irregularities affect sensitivity but not function.

  2. Prepare the electrodes. Cut two pieces of aluminium foil slightly smaller than the crystal faces. Optional: lightly draw a graphite layer over the crystal faces with a soft pencil to improve contact.

  3. Attach electrodes. Lay one piece of foil on the workbench. Place the crystal on it. Place the second foil on top. Gently press flat. The foil must make full, intimate contact with the crystal faces — any air gap reduces sensitivity.

  4. Attach wires. Fold a small tab of each foil out from one edge and attach a wire to each tab using a tiny dab of conductive epoxy, a crocodile clip, or by wrapping and taping. The two wires are your signal output — polarity is arbitrary at this stage.

  5. Optional — glue to a backing. For a more robust assembly, glue the crystal-foil sandwich to a thin flexible plastic sheet with a small amount of epoxy at one end only (leaving most of the crystal free to flex). This backing can be clamped at one edge while sound waves flex the free end.

  6. Seal and protect. Wrap the whole assembly loosely in cling film or place in a small container, with the wires coming out. This keeps moisture away from the crystal.

Testing

With a multimeter: Set to AC millivolts. Connect the two wires. Tap the crystal firmly with a finger — you should see brief voltage pulses of a few millivolts. Flex it gently and hold the flex: the reading may deflect and hold at a small DC offset. This confirms piezoelectric activity.

As a microphone: Connect the two wires to the instrument input of a guitar amplifier or audio interface. The input impedance must be high — at least 500 kΩ, ideally 1 MΩ or more. Low-impedance inputs (e.g., dynamic microphone inputs) will load the crystal and reduce sensitivity dramatically.

Speak or tap near the crystal. The amplified output should be clearly audible or visible as a waveform in recording software. The frequency response will be uneven and quality poor compared to a modern microphone, but the signal is unmistakably there.

Note

Improving sensitivity: The signal from a single slab is small. An op-amp configured as a unity-gain buffer (e.g., a TL071 or LM741 wired as a voltage follower, powered from a 9V battery) between the crystal and a standard audio input prevents impedance loading and can triple the usable output level.

The Science

Piezoelectric Effect (Summary)

When the crystal is deformed, the asymmetric charge distribution shifts:

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

Where \(\sigma\) is the surface charge density (C/m²), \(d\) is the piezoelectric coefficient (C/N for the converse, or V·m/N for the direct effect), and \(\varepsilon\) is the applied stress. Rochelle salt has \(d\) values around 170–350 pC/N depending on crystal orientation — roughly 100× larger than quartz’s ~2–3 pC/N.

Why Temperature Matters

The large \(d\) coefficient of Rochelle salt near room temperature arises because it is very close to its upper Curie temperature (≈24°C). Near any Curie temperature, the dielectric and piezoelectric susceptibilities peak sharply — the crystal is “on the edge” of a phase transition and responds strongly to mechanical perturbation. Quartz has no such nearby transition and therefore has a smaller but extremely stable and temperature-independent piezoelectric coefficient.

Safety

Note

Rochelle salt is used in food and medicine. It is non-toxic and requires no special handling.

Cream of tartar and baking soda are both food-grade. Dispose of waste solution down the drain — it is simply dilute sodium potassium tartrate.

Aluminium foil and standard epoxy are the only potentially irritating materials; handle epoxy with gloves.

Other Applications to Try

  • Phonograph pickup: Clamp the crystal assembly so a blunt stylus rests on it. Draw the stylus across the grooves of an old vinyl record by hand; the vertical modulation of the groove flexes the crystal, and a surprisingly recognisable signal can be heard through the amplifier.
  • Knock sensor: Mount the assembly firmly on a surface. Any vibration or tap on that surface produces a measurable pulse — a simple vibration or alarm detector.
  • Pressure measurement: Apply controlled weights to the crystal face and measure the output voltage; the piezoelectric coefficient can be measured and compared to literature values.
  • Growing with additives: Adding trace amounts of food colouring to the growth solution can tint the crystal without significantly altering its structure. Red, blue, and yellow crystals can be grown and compared.