The Art and Science of Crystal Growing

A Comprehensive Guide from Beginner to Intermediate

Learn to grow beautiful crystals at home with detailed techniques and troubleshooting

Crystal Growing Index

Crystals with Full Guide Sections

Chemical Color Crystal Shape Difficulty Notes
Potassium Alum Colorless Octahedral Easy Best beginner crystal - very forgiving
Ferric Ammonium Sulfate Pale violet Octahedral Easy Like alum with subtle color
Borax Colorless/white Prismatic clusters Easy Very fast - great for kids
Epsom Salt Colorless/white Needle clusters Easy Grows in hours, fragile
Copper Sulfate Deep blue Tabular/triclinic Medium Beautiful but needs sealing
Potassium Nitrate Colorless Long needles Medium Elegant needles, oxidizer
Cobalt Chloride Pink-red Prismatic Medium Color-changing, TOXIC
Potassium Chloride Colorless Cubic Medium-Hard Similar to salt
Sodium Chloride Colorless Cubic Hard Requires evaporation, slow

Additional Crystals (Brief Notes Only)

Chemical Color Growable? Notes
Cupric Chloride Blue-green Yes Hygroscopic, similar to copper sulfate
Ammonium Sulfate White Yes Forms prismatic crystals
Zinc Sulfate White Yes Similar to Epsom salt
Ferrous Sulfate Blue-green Challenging Oxidizes in air, turns yellow-brown
Silver Nitrate Colorless Challenging Light-sensitive, darkens, TOXIC
Ammonium Chloride White Yes (special) Sublimates - grows feathery crystals from vapor
Calcium Chloride White Not recommended Too hygroscopic - absorbs water from air

Quick Selection Guide

First crystal ever? → Potassium Alum or Borax

Want color? → Copper Sulfate (blue) or Cobalt Chloride (pink, toxic)

In a hurry? → Borax or Epsom Salt (hours, not days)

Want a challenge? → Sodium Chloride (perfect cubes take weeks)

Something different? → Ferric Ammonium Sulfate (violet alum) or Potassium Nitrate (needles)

Introduction to Crystal Growing

Crystal growing is one of the most rewarding and visually stunning areas of home chemistry. Unlike many experiments that happen in seconds, crystal growing teaches patience and careful observation while producing beautiful specimens you can keep forever. This guide will take you from your first simple crystals to understanding the science well enough to troubleshoot problems and optimize your results.

What Are Crystals?

A crystal is a solid material whose atoms, molecules, or ions are arranged in a highly ordered, repeating three-dimensional pattern called a crystal lattice. This regular arrangement extends in all directions, giving crystals their characteristic flat faces, sharp edges, and geometric shapes.

Every crystal belongs to one of seven crystal systems based on the symmetry of its internal structure:

Crystal System Shape Description Example Compounds
Cubic Equal axes, 90° angles NaCl, KCl, Alum, Ferric Ammonium Sulfate
Tetragonal Square base, different height Zircon
Orthorhombic Three unequal axes, 90° angles KNO\(_3\), sulfur
Hexagonal Six-sided symmetry Quartz, ice
Trigonal Three-fold symmetry Calcite
Monoclinic Tilted rectangular prism Borax, MgSO\(_4\)
Triclinic No right angles CuSO\(_4\)·5H\(_2\)O

The shape a crystal grows into is called its habit. While the crystal system is fixed by chemistry, the habit can vary based on growing conditions—the same compound might form needles, plates, or blocky crystals depending on temperature, impurities, and growth rate.

The Fundamental Principle: Supersaturation

All crystal growing relies on one key concept: supersaturation. A supersaturated solution contains more dissolved material than it can hold at equilibrium. This unstable state provides the driving force for crystallization.

There are three ways to create supersaturation:

1. Cooling Method

Most salts are more soluble in hot water than cold. By dissolving as much material as possible in hot water and then cooling, the solution becomes supersaturated.

Supersaturation by cooling: \[\text{Solubility}_{hot} > \text{Dissolved amount} > \text{Solubility}_{cold}\]

This method works best for compounds with steep solubility curves—those whose solubility changes dramatically with temperature.

2. Evaporation Method

As water evaporates, the concentration of dissolved material increases until it exceeds the saturation point. This method works for any soluble compound but requires patience.

Supersaturation by evaporation: \[\frac{\text{Dissolved mass}}{\text{Remaining water}} > \text{Saturation concentration}\]

This method is essential for compounds with flat solubility curves like sodium chloride.

3. Chemical Reaction

Sometimes supersaturation occurs when two solutions are mixed and produce a less-soluble product. This is the basis for precipitation reactions but rarely used for growing large crystals since growth is too rapid.

Nucleation vs. Growth

Crystal formation has two distinct phases:

Nucleation is the formation of tiny crystal “seeds” from a supersaturated solution. It can occur:

  • Spontaneously (homogeneous nucleation) when the solution is highly supersaturated
  • On surfaces (heterogeneous nucleation) on dust, scratches, or other imperfections
  • On seed crystals you deliberately introduce

Growth is the orderly addition of material to an existing crystal surface. For beautiful, large crystals, we want:

  • Slow nucleation: Fewer crystals forming means each one can grow larger
  • Steady growth: Consistent conditions produce clear, well-formed crystals

The key insight: mild supersaturation favors growth over nucleation, while high supersaturation favors nucleation over growth. This is why “more is better” when dissolving material usually backfires—you get many small crystals instead of a few large ones.

Getting Started: Essential Equipment and Techniques

Equipment

You need surprisingly little to grow excellent crystals:

Essential:

  • Clean glass containers (beakers, jars, or drinking glasses)
  • Thermometer (0–100°C range)
  • Stirring rod or clean spoon
  • Coffee filters or filter paper
  • Thread, fishing line, or thin wire
  • Pencil or stick to suspend crystals

Helpful:

  • Distilled or deionized water
  • Kitchen scale (0.1g precision is ideal)
  • Plastic wrap or paper towels for covering
  • Small containers for seed crystal selection
  • Tweezers or forceps

Advanced:

  • Constant-temperature bath or incubator
  • pH meter
  • Polarizing filters (to check crystal quality)

Water Quality

Tap water contains dissolved minerals, chlorine, and sometimes organic matter that can:

  • Inhibit crystal growth
  • Cause cloudiness
  • Create inclusions (trapped impurities)
  • Alter crystal shapes

Recommendation: Use distilled or deionized water for all crystal growing. Grocery-store distilled water works well and costs little.

Cleanliness

Dust, grease, and residue provide nucleation sites that cause unwanted small crystals. Before starting:

  1. Wash all containers with hot water and detergent
  2. Rinse thoroughly with distilled water
  3. Avoid touching the inside of containers
  4. Filter all solutions through coffee filters
  5. Cover solutions to keep dust out

The Basic Process

Regardless of which compound you’re growing, the process follows these steps:

Step 1: Prepare a Saturated Solution

  1. Heat water to the target temperature
  2. Add compound while stirring until no more dissolves
  3. Add a small excess (5–10%) and stir for several minutes
  4. Let undissolved material settle
  5. Filter the clear solution into a clean container

Step 2: Grow Seed Crystals

  1. Allow the saturated solution to cool slowly (or evaporate)
  2. Small crystals will form on the bottom and sides
  3. Select the best-formed, clearest crystal
  4. Remove it with tweezers
  5. Dry briefly on a paper towel

What makes a good seed crystal:

  • Clear, not cloudy
  • Well-defined faces and edges
  • No visible cracks or inclusions
  • Appropriate shape for the compound

Step 3: Prepare the Growing Solution

  1. Make a fresh saturated solution at an elevated temperature
  2. Filter into a very clean container
  3. Let it cool to your growing temperature
  4. The solution should now be mildly supersaturated

Step 4: Suspend and Grow

  1. Tie your seed crystal to thread or fishing line
  2. Attach to a pencil or stick laid across the container
  3. Lower the seed into the solution (don’t touch bottom or sides)
  4. Cover loosely to control evaporation
  5. Place in a location with stable temperature
  6. Wait!

Step 5: Maintain the Solution

As the crystal grows, it depletes the solution:

  • Every few days to weekly, remove the crystal
  • Make fresh saturated solution
  • Filter and cool to growing temperature
  • Return the crystal to the new solution

Understanding Solubility: The Key to Success

Solubility Curves

A compound’s solubility curve shows how much dissolves at each temperature. This is the most important data for crystal growing.

Compounds with Steep Curves (Best for Beginners)

These dissolve much more in hot water than cold, making supersaturation easy to achieve by cooling:

Compound 20°C 60°C 100°C Ratio (100°C/20°C)
Potassium Alum 5.9g 24.8g 357g 60x
Potassium Nitrate 31.6g 110g 246g 8x
Borax 4.7g 14.9g 40.1g 9x

Values are grams per 100mL water

Compounds with Moderate Curves

These require more care but still work well with cooling:

Compound 20°C 60°C 100°C Ratio (100°C/20°C)
Copper Sulfate 32.0g 64.0g 114g 3.5x
Epsom Salt 35.5g 50.2g 68.3g 2x

Compounds with Flat Curves (Challenging)

These require evaporation methods and patience:

Compound 20°C 60°C 100°C Ratio (100°C/20°C)
Sodium Chloride 35.9g 37.1g 39.1g 1.1x
Potassium Chloride 34.0g 45.5g 56.7g 1.7x

Calculating Supersaturation

The degree of supersaturation determines whether you’ll get many small crystals (high supersaturation) or controlled growth on your seed (low supersaturation).

Supersaturation ratio: \[S = \frac{C_{actual}}{C_{saturation}}\]

Where:

  • \(S = 1.0\): Exactly saturated (no growth)
  • \(S = 1.05\) to \(1.10\): Mild supersaturation (slow, steady growth)
  • \(S = 1.20\) to \(1.50\): Moderate supersaturation (faster growth, may get defects)
  • \(S > 1.50\): High supersaturation (spontaneous nucleation, many small crystals)

Example calculation for copper sulfate:

You want to grow at 20°C with mild supersaturation (S = 1.10).

  • Saturation at 20°C: 32.0g per 100mL
  • Target concentration: 32.0g × 1.10 = 35.2g per 100mL
  • This means: dissolve 35.2g in 100mL at 60°C (where 64g would dissolve), filter, and cool to 20°C

Temperature Stability

Temperature fluctuations are the enemy of crystal quality:

  • Warming: Crystals partially dissolve, creating pits and rough surfaces
  • Cooling: Excess material deposits unevenly, causing cloudiness
  • Cycling: Repeated warm/cool cycles create “growth rings” and inclusions

Temperature stability guidelines:

Crystal Quality Temperature Variation
Excellent ±0.5°C
Good ±1°C
Acceptable ±2°C
Poor ±5°C or more

Tips for temperature stability:

  • Grow in interior rooms (away from exterior walls)
  • Avoid windows and direct sunlight
  • Keep away from heating/cooling vents
  • Use larger volumes of solution (thermal mass)
  • Consider a water bath or insulated container

Troubleshooting Common Problems

Problem: Many Small Crystals Instead of One Large One

Causes:

  • Solution too supersaturated
  • Too many nucleation sites (dust, scratches)
  • Temperature dropped too quickly
  • Container not clean

Solutions:

  • Use lower supersaturation (dissolve less material)
  • Filter solutions more carefully
  • Use cleaner, smoother containers
  • Cool more slowly
  • Remove competing crystals as they form

Problem: Cloudy or Milky Crystals

Causes:

  • Trapped inclusions (liquid or gas bubbles)
  • Growth too fast
  • Temperature fluctuations
  • Impurities in solution

Solutions:

  • Reduce supersaturation (slower growth)
  • Stabilize temperature
  • Use purer water and chemicals
  • Filter solutions more thoroughly
  • Add crystal slowly to solution (don’t shock it)

Problem: Crystals Growing on Thread/Container Instead of Seed

Causes:

  • Seed crystal too small
  • Thread or container providing nucleation sites
  • Solution too supersaturated

Solutions:

  • Use a larger, better-formed seed crystal
  • Use smooth thread (fishing line rather than cotton)
  • Coat thread with clear nail polish or wax
  • Clean container more thoroughly
  • Reduce supersaturation

Problem: Crystal Dissolves Instead of Growing

Causes:

  • Solution is undersaturated (not enough dissolved material)
  • Temperature increased after adding crystal
  • Seed crystal has unstable surfaces

Solutions:

  • Make a more concentrated solution
  • Ensure temperature is stable or slowly dropping
  • Let solution cool to target temperature before adding seed
  • Select seed crystal with well-developed faces

Problem: Crystals Have Wrong Shape

Causes:

  • Growth too fast on certain faces
  • Impurities altering growth rates
  • Incorrect temperature
  • Stirring or vibration

Solutions:

  • Slower growth often improves shape
  • Use purer materials
  • Try different temperatures
  • Keep solution completely still
  • Different faces grow at different rates—this is sometimes unavoidable

Problem: Crust Forms on Solution Surface

Causes:

  • Evaporation at the surface causes local supersaturation
  • Surface cooling below bulk temperature

Solutions:

  • Cover container more completely
  • Use slower evaporation (smaller opening, cooler location)
  • Skim crust off periodically
  • Keep seed crystal below surface

Problem: Crystal Falls Off Thread

Causes:

  • Thread wasn’t embedded properly in seed
  • Crystal grew past the attachment point
  • Thread dissolved or weakened

Solutions:

  • Make a small groove in seed crystal for thread
  • Tie a small knot that the crystal grows around
  • Use fishing line or nylon thread (won’t dissolve)
  • Check and re-attach periodically

Specific Crystals: Properties, Techniques, and Challenges

The following sections cover specific compounds in detail, including their unique characteristics, optimal growing conditions, and common issues.

Potassium Alum (KAl(SO\(_4\))\(_2\)·12H\(_2\)O)

The Ideal Beginner Crystal

Potassium alum is widely considered the best compound for learning crystal growing. Its dramatic solubility curve, beautiful octahedral crystals, and forgiving nature make it perfect for beginners while still producing impressive results.

Properties

Property Value
Crystal System Cubic
Habit Octahedral (8-sided diamond shape)
Color Colorless, transparent
Hardness Relatively soft (2–2.5 Mohs)
Stability Excellent—stable in air

Solubility Data

Temperature Solubility (g/100mL)
0°C 3.0
10°C 4.0
20°C 5.9
30°C 8.4
40°C 11.4
50°C 17.0
60°C 24.8
70°C 40.0
80°C 57.4
100°C 357

The solubility increases by a factor of 60 from 20°C to boiling. This extreme change makes achieving supersaturation trivially easy.

Growth Rate

Under good conditions, alum crystals grow approximately:

  • 1–2mm per day with moderate supersaturation
  • 0.5–1mm per day with mild supersaturation

A crystal 3–4cm across takes 3–6 weeks.

Common Issues Specific to Alum

White coating or crust: Alum can partially dehydrate in dry air, forming a white powder (efflorescence). To preserve finished crystals, coat with clear nail polish or store in a sealed container.

Rounded edges: If the crystal is exposed to air while wet, surface tension can round the edges. Dry crystals quickly and completely.

Twinned crystals: Sometimes two crystals grow together at an angle. While scientifically interesting, they’re harder to grow large. Start with a single, untwinned seed.

Tips for Exceptional Alum Crystals

  1. Grow at cooler temperatures (15–18°C) for slower, clearer growth
  2. Use the lowest supersaturation that still gives growth
  3. Rotate the crystal 90° periodically for more symmetric growth
  4. Replace solution every 3–4 days for consistent supersaturation
  5. Be patient—rushing produces inferior crystals

Copper Sulfate (CuSO\(_4\)·5H\(_2\)O)

The Beautiful Blue Crystal

Copper sulfate produces stunning deep blue crystals with a more complex shape than alum. The color comes from copper ions coordinated with water molecules, making these crystals a conversation piece.

Properties

Property Value
Crystal System Triclinic
Habit Tabular to prismatic (flat parallelograms)
Color Deep blue (vibrant when fresh)
Hardness Moderate (2.5 Mohs)
Stability May effloresce in dry air

Safety Note: Copper sulfate is toxic. Do not ingest, wash hands after handling, and keep away from children and pets.

Solubility Data

Temperature Solubility (g/100mL)
0°C 23.1
10°C 27.5
20°C 32.0
30°C 37.8
40°C 44.6
50°C 53.8
60°C 64.0
80°C 87.6
100°C 114

The solubility increases by a factor of 3.5 from 20°C to boiling—moderate but workable.

Temperature Considerations

Copper sulfate’s moderate solubility curve means:

  • Temperature control is more important than with alum
  • Small temperature fluctuations cause noticeable effects
  • Growing at higher temperatures (25–30°C) gives higher effective supersaturation

Common Issues Specific to Copper Sulfate

Pale or dull crystals: Fresh copper sulfate crystals should be vibrant blue. Pale color indicates:

  • Impure starting material
  • Partial dehydration
  • Inclusion of bubbles or impurities

Solution: Use fresh, pure copper sulfate. Coat finished crystals to prevent dehydration.

White coating (efflorescence): In dry conditions, crystals lose water and develop a white-gray coating. Prevent by storing in sealed containers with a damp paper towel, or coat with clear lacquer.

Irregular shape: Triclinic crystals naturally have an asymmetric shape. Different faces grow at different rates depending on conditions. Accept this as characteristic of the compound.

Dissolving despite supersaturation: Copper sulfate dissolves more slowly than it grows. If you put a crystal into a barely supersaturated solution, it may dissolve initially before starting to grow. Wait or increase supersaturation slightly.

Tips for Exceptional Copper Sulfate Crystals

  1. Use very pure copper sulfate (reagent grade if available)
  2. Grow at stable temperatures—these crystals are sensitive
  3. Slow growth produces the best color and clarity
  4. Preserve immediately after drying with clear lacquer
  5. The triclinic shape means no face is exactly like another—appreciate the asymmetry

Borax (Na\(_2\)B\(_4\)O\(_7\)·10H\(_2\)O)

The Fast and Easy Crystal

Borax crystals form quickly and are perfect for impatient growers or projects with children. The crystals form dramatic clusters on any nucleation surface, making them ideal for “crystal ornaments” grown on pipe cleaners or other frames.

Properties

Property Value
Crystal System Monoclinic
Habit Prismatic, often in clusters
Color Colorless to white
Hardness Soft (2–2.5 Mohs)
Stability May effloresce slowly

Solubility Data

Temperature Solubility (g/100mL)
0°C 1.3
10°C 2.5
20°C 4.7
30°C 6.3
40°C 8.5
50°C 11.4
60°C 14.9
80°C 23.6
100°C 40.1

The solubility increases by a factor of 30 from 0°C to boiling, and by 8.5x from 20°C to boiling.

Growing Single Crystals

For larger, more distinct crystals:

  1. Make saturated solution at 60°C: 15g per 100mL
  2. Filter into clean container
  3. Cool to 20°C: S = 15/4.7 = 3.2 (still high)
  4. For slower growth, start from lower temperature
  5. Or: let solution sit until most excess crystallizes out, then pour off

Common Issues Specific to Borax

Opaque white crystals: Fast growth traps air bubbles. For clearer crystals:

  • Reduce supersaturation
  • Allow slower cooling
  • Use seed crystal method

Crystals crumble when dry: Borax crystals are fragile when freshly formed. Handle with care and let dry completely before moving.

Won’t crystallize: If solution cools without crystallizing, scratch the container bottom or add a dust particle to trigger nucleation. Borax can remain supersaturated for surprisingly long periods.

Tips for Borax Crystals

  1. The pipe cleaner method is foolproof for quick results
  2. Fuzzy or textured substrates provide excellent nucleation sites
  3. For larger single crystals, grow over several days with solution replacement
  4. Color can be added to the solution (food coloring doesn’t affect growth significantly)
  5. These crystals are perfect for kids—fast, dramatic, and relatively safe

Epsom Salt (MgSO\(_4\)·7H\(_2\)O)

The Crystal Needle

Epsom salt produces distinctive needle-like crystals (acicular habit) that grow very quickly. While individual crystals are less impressive than alum or copper sulfate, the rapid growth makes this compound excellent for demonstrating crystallization principles.

Properties

Property Value
Crystal System Orthorhombic
Habit Acicular (needle-like), prismatic
Color Colorless to white
Hardness Soft (2–2.5 Mohs)
Stability Efflorescent (loses water easily)

Solubility Data

Temperature Solubility (g/100mL)
0°C 26.9
20°C 35.5
40°C 45.8
60°C 54.6
80°C 64.2
100°C 68.3

The solubility only increases by a factor of 2.5x from 0°C to boiling. This relatively flat curve means:

  • Less extreme supersaturation from cooling
  • Evaporation method often works better
  • Temperature control less critical

Growing Larger Crystals

  1. Make saturated solution at 40°C: 46g per 100mL
  2. Cool to 20°C: S = 46/35.5 = 1.30
  3. Seed with a selected needle crystal
  4. Grow over several days

Epsom salt crystals tend to grow as bundles of needles rather than single large specimens. This is characteristic of the compound.

Common Issues Specific to Epsom Salt

Crystals fall apart: Epsom salt is highly efflorescent—it loses water to the air and crumbles. Finished crystals must be sealed immediately (clear lacquer or nail polish).

Clumps instead of single crystals: Needle-like crystals naturally cluster. For more distinct needles:

  • Lower supersaturation
  • More dilute solution spread thin
  • Select and grow single needles

Slushy appearance: If the solution is too concentrated, crystals form a wet mass rather than distinct needles. Use less material or more water.

Tips for Epsom Salt Crystals

  1. Quick results: pour thin layer on plate, crystals form in hours
  2. For demonstration, grow on dark paper to show needle structure
  3. Seal finished crystals immediately to prevent degradation
  4. Try growing on textured surfaces for interesting patterns
  5. These crystals photograph beautifully under side-lighting

Sodium Chloride (NaCl)

The Challenge Crystal

Table salt presents the greatest challenge for crystal growers. Its nearly flat solubility curve makes supersaturation difficult to achieve, and growth is extremely slow. However, the perfect cubic crystals are geometrically beautiful and demonstrate the fundamental principles of crystal growing.

Properties

Property Value
Crystal System Cubic
Habit Cubic (perfect cubes possible)
Color Colorless (pure)
Hardness Moderate (2.5 Mohs)
Stability Very stable, slightly hygroscopic

Solubility Data

Temperature Solubility (g/100mL)
0°C 35.7
20°C 35.9
40°C 36.4
60°C 37.1
80°C 38.0
100°C 39.1

The solubility only increases by 9% from 0°C to boiling. This means:

  • Cooling barely creates supersaturation
  • Evaporation is the only practical method
  • Growth is very slow (weeks to months)

Understanding the Challenge

With most compounds, you can dissolve excess material in hot water and cool to create supersaturation. With NaCl:

  • Saturated at 100°C: 39.1g per 100mL
  • Saturated at 20°C: 35.9g per 100mL
  • Cooling supersaturation: S = 39.1/35.9 = 1.09

This 9% supersaturation is so mild that crystals grow extremely slowly.

Hoppered Crystals

NaCl often forms “hoppered” crystals—cubes with stepped, pyramid-shaped indentations on each face. This occurs because:

  • Edges and corners grow faster than face centers
  • Creates a “staircase” pattern on each face

To encourage solid cubes instead of hoppered crystals:

  • Very slow growth (minimal supersaturation)
  • Extremely pure salt and water
  • Stable conditions

Common Issues Specific to Sodium Chloride

No crystals form: Even with evaporation, supersaturation may be too low. Increase evaporation rate or wait longer.

Many tiny crystals: Dust provides nucleation sites. Cover more carefully and filter solutions through fine filters.

Hoppered instead of solid: Accept this as characteristic of NaCl, or grow much more slowly with purer materials.

Crystals are small despite waiting: NaCl crystal growing is slow. A 5mm cube in 2–3 weeks is good progress.

Tips for Sodium Chloride Crystals

  1. Use pure salt (look for “canning salt” or “pickling salt”—no iodine or anti-caking agents)
  2. Set realistic expectations—this takes months for impressive specimens
  3. Control evaporation rate with covering and location
  4. Perfect cubes require very slow, careful growth
  5. The geometric perfection of well-grown NaCl crystals is worth the wait

Potassium Nitrate (KNO\(_3\))

The Needle Crystal

Potassium nitrate (saltpeter) produces long, elegant needle crystals with a distinctive orthorhombic shape. Its steep solubility curve makes it easier to grow than sodium chloride, though not as forgiving as alum.

Properties

Property Value
Crystal System Orthorhombic
Habit Prismatic to acicular (needle-like)
Color Colorless
Hardness Moderate (2 Mohs)
Stability Stable, not hygroscopic

Safety Note: KNO\(_3\) is a strong oxidizer. Keep away from organic materials and heat sources.

Solubility Data

Temperature Solubility (g/100mL)
0°C 13.3
10°C 20.9
20°C 31.6
30°C 45.8
40°C 63.9
50°C 85.5
60°C 110
80°C 169
100°C 246

The solubility increases by 8x from 20°C to boiling and by 18x from 0°C to boiling.

Crystal Shape Considerations

Potassium nitrate naturally grows as long needles. To encourage more stocky crystals:

  • Grow at higher temperatures
  • Rotate crystal periodically
  • Very slow growth with minimal supersaturation

Common Issues Specific to Potassium Nitrate

Extremely long, thin crystals: This is natural for KNO\(_3\). For thicker crystals, grow more slowly and at higher temperatures.

Crystals clump together: Multiple crystals nucleate and grow as bundles. Use very clean conditions and lower supersaturation.

Surface crust: Evaporation at the surface creates local supersaturation. Cover container more completely.

Tips for Potassium Nitrate Crystals

  1. The long needle shape is characteristic—appreciate it
  2. Very long crystals are fragile; handle carefully
  3. Grow at warmer temperatures (25–30°C) for thicker crystals
  4. Pure material is essential for clear crystals
  5. Store away from organic materials due to oxidizing nature

Ferric Ammonium Sulfate (NH\(_4\)Fe(SO\(_4\))\(_2\)·12H\(_2\)O)

The Iron Alum

Ferric ammonium sulfate (iron alum) is a double sulfate salt that forms beautiful octahedral crystals similar to potassium alum, but with a distinctive pale violet to lavender tint from the iron(III) ion. It’s an excellent alternative to regular alum for those wanting something slightly different.

Properties

Property Value
Crystal System Cubic
Habit Octahedral (like potassium alum)
Color Pale violet to nearly colorless
Hardness Soft (2–2.5 Mohs)
Stability Good—stable in air

Safety Note: Low to moderate toxicity. Wear gloves as solutions can stain skin yellow-brown.

Solubility Data

Iron alum has a similar steep solubility curve to potassium alum:

Temperature Solubility (g/100mL)
0°C ~12
20°C ~21
40°C ~40
60°C ~73
100°C ~180

Common Issues Specific to Iron Alum

Yellow instead of violet crystals: The violet color is subtle. Fresh, pure material gives the best color. Old or impure material may appear more yellow.

Rust-colored coating: If the crystal partially dehydrates or oxidizes, it may develop brown spots. Store finished crystals sealed.

Tips for Ferric Ammonium Sulfate Crystals

  1. Grows almost identically to potassium alum—use same techniques
  2. The violet color is best seen in thick crystals under good lighting
  3. Solution appearance (yellow-brown) doesn’t reflect final crystal color
  4. Can be grown alongside potassium alum to compare crystal habits
  5. Good intermediate project after mastering regular alum

Cobalt Chloride (CoCl\(_2\)·6H\(_2\)O)

The Color-Changing Crystal

Cobalt chloride produces striking pink to red crystals when hydrated. The unique feature is their dramatic color change with humidity—blue when dry, pink when hydrated. This makes them both beautiful specimens and functional humidity indicators.

Properties

Property Value
Crystal System Monoclinic
Habit Prismatic to tabular
Color Deep pink-red (hydrated), blue (anhydrous)
Hardness Soft (2 Mohs)
Stability Hygroscopic—absorbs water from air

Safety Note: TOXIC and classified as a carcinogen. Wear gloves at all times. Do not ingest. Work in well-ventilated areas. This compound is for experienced growers who understand and respect chemical hazards.

Solubility Data

Temperature Solubility (g/100mL)
0°C ~33
20°C ~52
40°C ~77
60°C ~98
100°C ~115

Moderate solubility curve—cooling method works but not as dramatically as alum.

The Color Change

Cobalt chloride’s color change is due to coordination chemistry:

  • Pink (CoCl\(_2\)·6H\(_2\)O): Cobalt coordinated with six water molecules
  • Blue (anhydrous CoCl\(_2\)): Cobalt coordinated with chloride ions

You can demonstrate this reversibly: 1. Gently heat a crystal—it turns blue 2. Expose to humid air or add water—it turns pink again

Preserving Cobalt Chloride Crystals

The hygroscopic nature makes preservation critical:

  1. Dry crystal carefully
  2. Seal immediately with clear lacquer (multiple coats)
  3. Store in sealed container with desiccant
  4. Without sealing, crystals will absorb moisture and may deliquesce (dissolve in absorbed water)

Common Issues Specific to Cobalt Chloride

Crystals dissolve themselves: In humid conditions, crystals absorb enough water to partially dissolve. Work in dry conditions and seal immediately.

Blue color on surface: Surface dehydration. The inside is still pink. Either seal immediately or rehydrate in humid environment.

Poor crystal shape: Cobalt chloride crystals can be irregular. Slower growth and careful seed selection help.

Tips for Cobalt Chloride Crystals

  1. Only attempt after mastering safer compounds
  2. Always wear gloves—solutions stain and are toxic
  3. The color-changing property makes these conversation pieces
  4. Seal thoroughly to maintain the pink hydrated form
  5. Can make humidity indicator paper by soaking filter paper in dilute solution
  6. Consider this an advanced project due to toxicity

Other Inventory Chemicals: Brief Notes

The following chemicals from the inventory can form crystals but present additional challenges. Brief notes are provided for experimenters who want to try them.

Cupric Chloride (CuCl\(_2\)·2H\(_2\)O)

Color: Blue-green | Difficulty: Medium-Hard

Copper(II) chloride forms attractive blue-green crystals but is hygroscopic (absorbs moisture). Grows similarly to copper sulfate but less forgiving. Seal finished crystals immediately. Moderately toxic—wear gloves.

Ammonium Sulfate ((NH\(_4\))\(_2\)SO\(_4\))

Color: Colorless/white | Difficulty: Medium

Forms prismatic crystals. Moderate solubility curve allows cooling method. Solutions are mildly acidic. Less commonly grown but produces decent crystals. Keep away from bases (releases ammonia).

Zinc Sulfate (ZnSO\(_4\)·H\(_2\)O)

Color: Colorless/white | Difficulty: Medium

Similar to Epsom salt in behavior. Forms prismatic crystals. Moderate toxicity—wear gloves. Solubility curve allows cooling method. Less dramatic than other options.

Ferrous Sulfate (FeSO\(_4\)·7H\(_2\)O)

Color: Blue-green (fresh) | Difficulty: Hard

The challenge: ferrous sulfate oxidizes in air, turning from blue-green Fe²⁺ to yellow-brown Fe³⁺. Crystals may develop brown coating. Work quickly, minimize air exposure, and consider adding a small amount of iron metal to the solution to maintain the reduced state. Interesting for demonstrating oxidation chemistry.

Ammonium Chloride (NH\(_4\)Cl)

Color: White | Difficulty: Special

Ammonium chloride sublimates (solid → gas → solid) when heated, allowing crystal growth from vapor rather than solution. Heat crystals gently in a flask—feathery, fern-like crystals deposit on cooler surfaces. A completely different technique from solution growing.

Procedure: 1. Place ammonium chloride crystals in a dry flask 2. Heat gently from below 3. Watch white crystals deposit on upper, cooler surfaces 4. Creates beautiful dendritic (tree-like) patterns

Silver Nitrate (AgNO\(_3\))

Color: Colorless | Difficulty: Hard

Silver nitrate is light-sensitive—crystals and solutions darken when exposed to light as Ag⁺ reduces to metallic silver. Must be grown in the dark or under red safelight, and stored in opaque containers. Also toxic and causes permanent black skin stains. Only for experienced growers. Interesting for photography enthusiasts.

Advanced Techniques

Controlling Crystal Shape

Crystal shape (habit) can be modified by:

Growth Temperature

Higher temperatures often produce different habits:

  • Alum: Still octahedral, but faces may differ
  • KNO\(_3\): Stockier at higher temperatures
  • CuSO\(_4\): Thicker tablets at higher temperatures

Impurities and Additives

Small amounts of certain chemicals can dramatically alter crystal shape:

  • Borax in NaCl solution: Produces octahedral instead of cubic crystals
  • Urea in KNO\(_3\) solution: Changes needle length
  • Organic dyes: May alter habit and add color

Warning: Additives usually reduce clarity and can create unexpected effects. Experiment carefully.

Growth Rate

Faster growth tends to produce:

  • Needles or dendrites (branching structures)
  • Skeletal or hoppered crystals
  • Surface defects

Slower growth tends to produce:

  • More equidimensional crystals
  • Solid, well-formed faces
  • Better transparency

Growing Very Large Crystals

To grow crystals larger than a few centimeters:

  1. Start with an excellent seed: Clear, well-formed, no defects
  2. Use minimal supersaturation: S = 1.05 to 1.10
  3. Control temperature precisely: ±0.5°C or better
  4. Replace solution regularly: Every 2–3 days
  5. Rotate the crystal: Promotes symmetric growth
  6. Be patient: Months, not weeks
  7. Remove secondary crystals: Any that nucleate on the container

Very large crystals (10+ cm) can take 6 months to a year and require dedicated temperature control.

Preserving Finished Crystals

Different compounds require different preservation methods:

Compound Stability Preservation Method
Alum Moderate Clear nail polish or display case
Ferric Ammonium Sulfate Moderate Clear lacquer, sealed container
Copper Sulfate Poor (efflorescent) Clear lacquer + sealed container
Cupric Chloride Poor (hygroscopic) Multiple lacquer coats + sealed container
Cobalt Chloride Poor (hygroscopic) Multiple lacquer coats + sealed container + desiccant
Borax Moderate Clear lacquer
Epsom Salt Poor (efflorescent) Immediate lacquer coating
Zinc Sulfate Poor (efflorescent) Immediate lacquer coating
Ammonium Sulfate Moderate Clear lacquer
Ferrous Sulfate Poor (oxidizes) Lacquer immediately, store in inert atmosphere
Silver Nitrate Poor (light-sensitive) Store in dark, opaque container
Ammonium Chloride Good Sublimes if heated, otherwise stable
Sodium Chloride Good Optional coating, store dry
Potassium Chloride Good Optional coating, store dry
Potassium Nitrate Good Optional coating

Lacquering technique:

  1. Ensure crystal is completely dry
  2. Apply thin coat of clear lacquer, nail polish, or acrylic spray
  3. Let dry completely
  4. Apply second coat
  5. Avoid thick coatings that obscure crystal detail

Temperature Control Methods

For serious crystal growing:

Passive Methods

  • Location selection: Interior rooms, closets, basements
  • Thermal mass: Large volumes of solution, insulated containers
  • Timing: Grow in seasons with stable temperatures

Active Methods

  • Constant-temperature bath: Container in larger water bath
  • Aquarium heater: Inexpensive and adjustable
  • Incubator: Laboratory-grade temperature control
  • Evaporative cooling: For hot climates

Reference Tables

Solubility Comparison (g per 100mL water)

Compound 0°C 20°C 40°C 60°C 100°C
Potassium Alum 3.0 5.9 11.4 24.8 357
Ferric Ammonium Sulfate ~12 ~21 ~40 ~73 ~180
Copper Sulfate 23.1 32.0 44.6 64.0 114
Cupric Chloride ~43 ~71 ~85 ~98 ~120
Cobalt Chloride ~33 ~52 ~77 ~98 ~115
Borax 1.3 4.7 8.5 14.9 40.1
Epsom Salt 26.9 35.5 45.8 54.6 68.3
Zinc Sulfate ~42 ~54 ~67 ~75 ~81
Ammonium Sulfate ~70 ~75 ~81 ~88 ~103
Ferrous Sulfate ~16 ~26 ~40 ~55 decomposes
Sodium Chloride 35.7 35.9 36.4 37.1 39.1
Potassium Nitrate 13.3 31.6 63.9 110 246
Potassium Chloride 27.6 34.0 40.0 45.5 56.7

Difficulty Comparison

Compound Difficulty Best Method Time for 1cm Crystal
Borax Easy Cooling 1–2 days
Alum Easy Cooling 1–2 weeks
Ferric Ammonium Sulfate Easy Cooling 1–2 weeks
Epsom Salt Easy Evaporation 2–3 days
Copper Sulfate Medium Cooling 1–2 weeks
Cobalt Chloride Medium Cooling 1–2 weeks
Potassium Nitrate Medium Cooling 1–2 weeks
Ammonium Sulfate Medium Cooling 1–2 weeks
Zinc Sulfate Medium Cooling/Evap 1–2 weeks
Cupric Chloride Medium-Hard Cooling 1–2 weeks
Potassium Chloride Medium-Hard Evaporation 2–4 weeks
Ferrous Sulfate Hard Cooling 1–2 weeks
Sodium Chloride Hard Evaporation 3–6 weeks
Silver Nitrate Hard Cooling (dark) 1–2 weeks
Ammonium Chloride Special Sublimation Minutes

Conclusion

Crystal growing combines science, patience, and artistry. The principles are simple—supersaturation drives crystallization—but achieving exceptional results requires attention to detail and understanding of the specific characteristics of each compound.

Start with potassium alum or borax to learn the basics. Progress to copper sulfate for beautiful colored crystals. Challenge yourself with sodium chloride or large single crystals of any compound.

Above all, be patient. The best crystals are grown slowly, with careful control of temperature and concentration. Keep notes on your procedures and results—crystal growing is a skill that improves with practice and careful observation.

Happy growing!

Further Resources

Books:

  • Crystals and Crystal Growing by Alan Holden and Phylis Morrison (the classic text)
  • The Art of Growing Crystals edited by J.J. Gilman

Websites:

  • CrystalVerse (crystalverse.com) - Detailed guides for many compounds
  • Crystal Growing Wiki - Community knowledge base
  • Royal Society of Chemistry education resources