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Crystals - a handbook for school teachers

Elizabeth A. Wood, 1972

Written for the Commission on Crystallographic Teaching of the International Union of Crystallography


To the teachers of young children everywhere:

This booklet was produced for the Commission on Teaching of the International Union of Crystallography, which is an organization for the benefit of the science of crystallography throughout the World. It is not a trade union, but a group of people of all nations interested in crystals.

Many teachers have found that children are interested in crystals. There are good reasons for the teacher to encourage this interest. Children can perform simple experiments with crystals and so get the feel of doing science themselves, the experience of watching something happen in their own experiments. Crystals are of interest to chemists, physicists, geologists, biologists and mathematicians. To study crystals is to be part of all these fields and to become aware that Nature is not separated into chemistry, physics, geology, and biology.

Most teachers at the present time (the 1970s) did not learn about crystals when they were in school and college. The purpose of this booklet is to give them some background of understanding of crystals so that they can enjoy working with children who are interested in crystals. It is not a systematic course in crystallography. This would not be suitable. Those students who want to know that much about crystallography will take courses in crystallography at the university. It is a handbook for your enjoyment.

In this booklet, technical terms are avoided as much as possible, not to make it easier, but to avoid the pitfall of substituting learning names for thinking about what is going on. Children think they know why an apple falls because they have learned the word `gravity', but our most competent scientists are puzzled by the way in which an apple and the Earth are drawn toward each other.

Most books on crystallography, the subject that deals with the study of crystals, emphasize the importance of symmetry in the classification of crystals. However, many of the crystals that children can grow themselves, or find in nature, have shapes that do not exhibit perfect symmetry, because the growing conditions were not the same all around the crystal. It takes a mature imagination and experience with some perfectly symmetrical crystals to imagine what such a crystal would have looked like if the growing conditions had been uniform.

Unless students can be convinced from their own observations that symmetry is really useful in classifying crystals, there is no merit in having them memorize the symmetry terms, since they have no meaning for them. The essence of science is observation and wonder, curiosity and the effort to satisfy that curiosity. Learning what others have found out is part of learning about science, but first we must see how scientists learn what they know about nature so that we may be convinced that their results are based on repeatable experiments.

For these reasons this booklet does not deal with systematic crystallography, the classification of crystals according to their symmetry. It seeks to lay a firm foundation for later study of crystallography by encouraging observation and experimentation. Over a period of time, the students' observations will probably lead them to conclusions such as the following:

1. Under suitable conditions some kinds of solid matter form in shapes called crystals.

2. Crystals grow bigger by adding more layers of solid matter around their outsides.

3. Crystals form from solution when the solvent evaporates. Crystals form from the molten state when the liquid cools. Crystals form from warm invisible vapor when that vapor meets a cooler surface.

4. Crystals of different substances have different forms.

5. Crystals of different substances have different properties; that is, some are colored and some are not; some grow nicely and some do not; some have cleavage (to be discussed later) and some do not; some look bright between crossed polarizers (see section III-D) and some do not.

6. (for older students) There must be something orderly about the way a crystal forms that is responsible for its flat faces, its characteristic shape, and the way it affects light. This orderliness must be different for different substances.

If your students have some conviction about such conclusions from their own observations, they will have a good foundation in the science of crystallography.

* * *

A crystal of a given substance or material shows plane faces always at the same angles to each other and has its other orderly properties because it is made up of atoms, ions, or molecules arranged in a very orderly way. This orderliness of structure is found in almost all solid matter, though some substances have a more orderly arrangement than others. Even in wood the molecules are arranged in good order along the fibers, though there is not much orderliness from one fiber to the next. Is wood, then, a crystal? It doesn't show shiny faces. Some crystallographers (people who study crystals) would say its fibers are crystals; some would not.

A substance that is made up of crystals is called a crystalline substance. Sometimes the word polycrystalline is used to indicate a substance made up of many crystals. In a single crystal, the orderliness of rows of atoms is not interrupted and does not change direction. When two crystals grow against each other, the boundary between them marks the place where the orderly array of one makes an angle with the orderly array of the other. A slice through four crystals with such boundaries is crudely suggested by this sketch. The solid lines represent boundaries between crystals (sometimes called grain boundaries). The dotted lines represent layers of atoms, ions, or molecules.

Many substances that we are familiar with are made of very orderly crystals that do not show their bright faces because neighboring crystals have grown against each other with irregular boundaries. Nearly all rocks are made up of crystals and the different kinds of crystals in a rock can often be distinguished from each other. Metal objects are made up of interlocking crystals. Sometimes their boundaries can be seen, as in the zinc coating on galvanized iron often used for pails (buckets) and garbage cans. Sometimes a brass door handle shows the boundaries between the crystals of which it is composed.

A substance in which the atoms or ions or molecules are not arranged in orderly rows is called a glass. Window glass is a familiar example. Volcanic glass, and some volcanic ash, is also glassy - not crystalline. There is a glassy kind of candy that is very brittle, usually made with nuts in it. It is made by cooling the melted sugar very quickly so that crystals do not have time to grow before the fluid gets too stiff to allow the molecules to move about and take their proper positions to make a crystal. This suggests that other glasses may be formed by quick cooling. This is so of volcanic glass and of some manufactured glass too, although glass manufacturers have learned to produce mixtures that can be cooled at a convenient rate without crystallizing. In some very old glass, made before the art was well developed, crystals have begun to form as, over the years, the atoms have slowly migrated into orderly positions, drawn by their attraction for each other. There are no very old volcanic glasses, geologically speaking. In hundreds of thousands of years, the atoms have had time to get together to form crystals.

Young people learn best by doing, not be being told. The best way for a child to learn about crystals is by experience, not by having someone tell him about them. Let him observe and wonder and ask questions. Then perhaps you can help him seek answers to them. We will not even try to define the word crystal until we have had some experience with crystals. It is essential that you, the teacher, have these experiences yourself so that you can enjoy the discoveries with your students.

The rest of this handbook will be written as though for the students. If it teaches something that you already know, remember that it was written for all schools, everywhere in the World.

Equipment and materials

A. Essential

Salt (table salt, sodium chloride, NaCl)Cup, glass or other container
Sugar (sucrose or saccharose, C12H22O11)Measuring cup (8 ounce; about 235 cc)
 Thread or thin string

B. Desirable

Borax (Na2B4O7.7H2O)Magnifying glass
Alum (ammonium alumTweezers or forceps
NH4Al(SO4)2.12H2O or potassium alum, KAl(SO4)2.12H2O)Microscope slides (glass). The bottom of an overturned drinking glass can be used.
Copper sulfate (blue vitriol, CuSO4.5H2O)Candle or match flame
 Source of heat to boil water
Epsom salt (MgSO4.7H2O)Refrigerator or temperature below 0° C
Salol (phenyl salicylate, HOC6H4COOC6H5)Two pieces of polarizing film, such as Polaroid
Bismuth (Bi) 
Naphthalene (moth flakes, C10H8) 

Crystals in classroom and home

A. Growing crystals from solution

1. Salt (table salt, sodium chloride, NaCl)

We will begin with salt because everybody has it. While the salt experiment is going on, you can be getting together the materials for the later experiments.

a. Growing the crystals and observing their growth.

Put 3 teaspoonfuls of salt into 1/3 cup of water. Stir well. Most of the salt will dissolve, forming a solution of salt in water, but some will remain in the bottom of the container and the solution may appear cloudy. (Some producers of salt coat the salt grains with a harmless insoluble substance so that they will not stick together in damp weather. The following procedure is used to separate this and the undissolved salt, if any, from the salt solution.) Let the mixture stand over night. Next morning, the solution will appear clear. Pour the clear solution into a shallow glass or cup, being careful not to stir up any of the material from the bottom. (This process of separating a liquid from a solid, simply by pouring the liquid off, is called decanting.) In a chemical laboratory, amounts would be given in grams. Here we use the measuring cup and the measuring teaspoon familiar to all cooks. A cup holds 8 fluid ounces or approximately 236 milliliters. A teaspoonful is closely equivalent to 5 milliliters.

Discard the solid material. Let the clear solution stand, uncovered, for a few days. To keep the dust out, it may be well to place an overturned box over it.

A given amount of any solvent, such as water, can hold in solution just a certain amount of a particular substance. When it has this amount in solution, it is said to be a saturated solution of that substance. If it has less than that amount, it is undersaturated. In some cases, substances seem to need a nucleus - a tiny bit of crystal of their own kind - to cause the beginning of crystallization of the solid from the solution. In such cases, as a saturated solution stands, and the solvent evaporates, it may become super-saturated, containing in solution more than it would if it were in contact with crystalline material of the substance that it has in solution. In such cases, addition of the tiniest fragment of the solute (the dissolved substance) will cause precipitation of the excess solute in the bottom of the vessel.

When the first solid particles appear in the bottom of your salt solution, examine them with a magnifier. Try to pay attention to one particular particle and watch it change from day to day. If the vessel containing the solution is glass, you can place it over a piece of paper on which a marked circle helps you locate the particle you are watching. (A white crust may form at the edge of the solution, where evaporation is rapid. We will discuss that later.) You may find tht a strong light from the side helps your observations.

The particles gradually forming from solution are salt crystals. If you will watch the very early stages of their formation you will find that they look square. If you look at them from the side, you will see that they also look square, or perhaps rectangular. Their sides are very precisely at right angles to each other and stay that way as they grow.

Think about this! Out of that formless solution come these perfectly formed solid shapes, whether you are evaporating the solution in Spain or in Siberia, in Africa, America, or Australia, in a submarine or in an airplane. Dependably, the solid that comes out of salt solution forms little crystals with bright, shiny faces at right angles to each other. How do you suppose it does this?

Take one of the little crystals out of the container, with tweezers if you have them, and dry it off. You can put it in a box and keep it. It will not change, unless the weather is very damp. With extreme dampness, water from the air may gather on the crystal and dissolve it. Those left in the container continue to grow larger because, as the water evaporates, the salt that you dissolved in it has to come out of solution and it is added to the tiny salt crystals, making them bigger. As layer after layer is added, like layers of paint on a box, each flat face just progresses outward, staying precisely at right angles to its neighbors.

What happens when two crystals that are growing next to each other meet? Watch carefully and see. In most cases they grow together, irregularly, while their beautifully flat faces continue to grow outward on the free sides where they are not in contact with each other. When they have grown together for a while, pick them up with the tweezers. Can you tell where one crystal ends and the other begins? In some cases this is easy, in other cases not. Can you pull them apart?

As your solution evaporates further, many crystals will grow together in the bottom of the container. The white crust at the side is formed of just such crystals grown together but they are very very small. Where the evaporation was very rapid, many crystals started to grow at the same time and quickly met neighboring crystals so that none could grow large.

In the very small spaces between some of these crystals and between this crust of crystals and your container capillary action causes the solution to be drawn up the side of the container where it then evaporates rapidly and more white crust is formed.

How could you grow a bigger crystal that still had its perfect shape because it had not come in contact with a neighboring crystal? Try to answer this before reading further.

Here are two methods to try:

1. Since rapid evaporation caused many crystals to start growing at the same time, close to each other, perhaps we could cause fewer crystals to grow, farther apart, by preventing rapid evaporation. We could put a cover on the container - not a tight cover, that would stop the evaporation completely, but more of a cover than the overturned box. A piece of paper or cloth could be fastened over the top of the container with a rubber band to let evaporation proceed slowly.

2. We could take one good little crystal out, transfer the saturated solution to another container, and put that crystal back in again. Maybe it would be the only one to grow. A crystal used in this way is called a seed crystal. (Remember that any solution, clinging to the crystal you take out or to the tweezers or your fingers, is saturated. As it evaporates little crystals will grow rapidly in it and form additional seeds that will compete for material with the one that you want to grow. For this reason you should dry your crystal quickly on cleansing tissue or a clean handkerchief, and wash and dry the tweezers and your fingers.)

When a crystal grows, resting on a surface, the part next to the surface is deprived of additional material and cannot develop. To let a crystal develop on all sides, we must hang it from a thread in the solution. Tying a thread around a very small crystal is not easy. An alternative method is to cement the thread to the crystal with a tiny bit of the kind of cement that is used for mending plates. Let it dry over night before hanging the crystal in the saturated solution.

b. What to do with the crystals

1. An instructive exhibit could be made by taking out a series of crystals (with tweezers) at various stages of growth, and mounting them with a very small bit of glue or cement on a card or paper (perhaps black or dark colored). The sequence, from small to large, will show how the crystals keep the same shape as they grow larger.

Try to avoid choosing a piece made up of more than one crystal, since this makes observation of the shape and comparison of the sizes more difficult.

Two crystals

2. Break them. Tap a crystal lightly with a small hammer or the heavy handle of a knife or screwdriver or the bowl of a spoon. It will break along plane surfaces parallel to the plane faces that form the outside surface.

This may break into ... these   

These may be broken into still smaller pieces, again with plane surfaces (that shine brightly in a strong light) parallel to the original ones. They may be broken anywhere at all. The surfaces will still be parallel to each other.

This tendency of a crystal to come apart along plane surfaces of a particular orientation is known as cleavage. Not all crystals show cleavage. Some just break like a piece of glass.

In salt, the cleavage planes are parallel to the growth faces, the faces that form the outer surfaces as the crystal grows. In crystals of some other substances, the cleavage planes are not parallel to the growth faces.

What is it that makes sodium chloride crystals grow in rectangular shapes and come apart along planes at right angles to each other? Crystallographers wondered and speculated about such questions for many years. Only in the twentieth century has the use of x-ray diffraction enabled us to find out how the arrangement of atoms and ions and molecules in crystals accounts for the way they grow and can be cleaved and for many other properties. The x rays do not cast shadows of the atoms as they do of the bones of the body. The atoms are too small for that. The x rays are scattered by the atoms, and by studying the directions in which they are scattered crystallographers learn how the crystals are put together.

In sodium chloride, common table salt, they find that the sodium and chlorine ions alternate with each other, like this.

It would take 1017 blocks like this to make one cubic grain of salt 1 mm on each edge. That is 100,000,000,000,000,000. That's a hundred million times a thousand million. You wold expect such an arrangement to build rectangular crystals. You might also expect that it would come apart most easily along those layers of sodium and chlorine ions.

Each crystal has its own characteristic arrangement of atoms, ions, or molecules which accounts for the shape in which it grows and for its other properties.

3. Keep a few of the best crystals in a small box or jar or envelope, for use in later experiments. (See experiments with polarized light, Section III-D.)

4. Place a crystal on a glass slide or other clean surface and place a large drop of water on it, watching it with a magnifier as it dissolves in the water. The corners get rounded quickly because they have three faces exposed to the solvent. The edges get rounded somewhat less quickly because they have only two faces exposed to the solvent. If the crystal is rescued before it has dissolved completely, is dried with cleansing tissue or a clean handkerchief, and then placed again in the saturated salt solution, it will start to grow again, filling out the edges and corners and regaining its original shape!

5. You can use any of your crystals as seed crystals, to grow bigger crystals from a saturated solution of the same substance, but the smaller ones are better.

6. Use your crystals as source material to start over again from the beginning. They are now pure salt, without any insoluble coating.

c. Lessons from this section

An important lesson to be learned from this section is that a salt crystal grows by adding salt to itself from the water solution of salt that surrounds it and that it grows with flat shiny faces which are at right angles to each other, provided its growth is not obstructed. The fact that these crystals have cleavage indicates that, within the crystal

Earlier this week we talked about how to write for your website so that people will want to read it. But what about the way your text actually appears on the page?

Words aside, text is yet another visual element on your website, just like your images, colors, and template.

And just like those other visual elements, text needs to be arranged and styled in a way that makes it clear and accessible to people who visit your website. For website text, your mantra should be “readability, readability, readability.”

Focus on text readability

“Readability” is basically a measure of how easy it is for people to recognize words, sentences, and paragraphs. Considering how quickly people glance at your website, readability becomes a crucial factor in how long they will spend on your page and whether they will even attempt to read what you have to say.

Reading on screens is tough. Different studies have found that people read slower, less accurately, and remember less of what they read when it’s presented on a screen. By some estimates, website visitors only read about 25% of the text on your site.

Any obstacle to readability—text that is too small, a font color that’s hard to read, or irregular line lengths—will lower that percentage even more.

So the more you can do to keep your text readable, the happier your visitors will be and the more likely they’ll want to stick around a while.

How do you design text that will catch—and hold—readers’ eyes? Here are some factors to look at:


1. Line length

We may have grown up reading from 8.5 x 11 sheets of paper, but when it comes to on-screen reading, line length needs to be much shorter. With long lines of text, people’s eyes have to move side-to-side to take in all the words. It’s harder to skim, and harder to keep your place as you move from one line down to the next. Just compare these two websites, with the same text and font (16 pt Noto Sans):

In the first example, the line length is 140 characters. In the second example, I added a Columns element and put the text on one side and the photo on another. This shortens the line length to 67 characters.

Readers see a longer line of text as more of an investment to read, so they are more likely to skip over it. The same amount of text on shorter lines looks more digestible and inviting.

For basic landing page text, there are lots of estimates for proper line length, but most fall somewhere in between 40-70 characters long, depending on the font size you are using.

With blogs, the lines can be longer because people dive deeper into the content. For example, the line length of this blog is about 115 characters. To figure out the length of your own lines of text, copy and paste it into a tool like lettercount.com.

Note: Many Jimdo templates have full-width content areas (like template Zurich in the example above), which make them ideal for showing large images or photo galleries. When it comes to landing page text, though, it’s best not to let your lines run across the entire width. Try using Columns elements and Photo elements to break up the text and create shorter lines.


2. Contrast

The better the color contrast between the background and the text, the better the readability.

For your website, make sure there’s enough difference between your background color and your text color so that the text is crystal clear. Even very different colors can clash badly when put together (like the red and blue example, below).

Choosing the right colors

With Jimdo’s Style Editor, the sky is really the limit when it comes to choosing colors for your fonts and backgrounds. Just make sure that you don’t sacrifice readability for creativity. As one of Jimdo’s web designers puts it, “There’s a reason books have been printed since time immemorial with black ink on white paper.” If black and white seem too unexciting for you, you can always try gray, navy, brown, or another dark color.

Make sure that the contrast between your background and your text is strong enough that text is easy to read. Some color combinations like red and blue are particularly hard to read.

Light text on dark backgrounds

You can go the other way and choose a dark background with light text, but this is best used in small doses. Even with sufficient contrast, this combination can be tiring on the eyes if you have a lot of text to read. If you’re going this route, you need to make the font larger to get the same readability of a dark font on a light background.

Light text on a dark background can look nice in small doses, but the combination can strain the eye if there’s too much text, or if the text is too small.

Link colors

You can also adjust the color of your links using the Style Editor. Choose a color that complements your text but that will still stand out so people can recognize a link when they see it.

Not sure about your color combinations? It always helps to get a second opinion. If you use Jimdo’s Style Editor to choose a color scheme, it’s a safe bet that you’ll start with a color combination that works well. You can also use online web accessibility tools that measure the contrast for you and tell you if you’re in an acceptable range.


3. White space

Squished line height like this is tough to read.

On your website, there’s no need to crowd. Give your text a comfortable amount of “white space” (empty space) around it and it will be much easier to read and scan.

Think about it—if you open up a webpage and just see a big block of text, you’ll probably not even bother to read any of it. But if you see text broken up into smaller, more manageable pieces, you’ll probably give it a glance at least.

Add space and break up paragraphs

To create more white space on your site, try using the Spacing element and the Horizontal Line element to add more cushioning between Text elements.

You can also break up text into smaller paragraphs, or add bulleted/numbered lists.

Adjust your line height

Use the Style Editor to increase the line height of your body text (this adjusts the amount of white space between lines of text). There’s no one rule for what your line height should be—you’ll need to adjust it based on the what looks best with the font and font size you’re using.

You can adjust the line height of your paragraph text in Edit Mode.

In the example website below, both the images and the text have a nice amount of empty space around them. This creates a sense of harmony and visual hierarchy (you know what to look at and read first). Notice how the designer has increased the line height in the text on the right hand side, which makes the smaller font easier to read.


4. Alignment and “rough edges”

In Text elements, you’ll see that you have the option of making text aligned left, right, center, or justified.

Choose the correct alignment

Left aligned text is pretty much the gold standard. Steer clear of center- or right-alignment for body text. While centered text can work well for titles and headings, it’s tough to read more than a few lines of it. That’s because the starting place of each line of text changes with every line, meaning that readers have to search to find the beginning of each line.

Left aligned text is easier to read than center aligned text, because each line starts at the same place. With center alignment, the reader has to search for the beginning of each line.

Justifying your text can make for very neat, uniform paragraphs. But depending on your text, sometimes the words can appear stretched or unnatural-looking.

Neaten up your right edge

If you’ve been adjusting the styles and fonts sizes of a Text element, you may find that even if it’s left-aligned, the right hand side has gotten a little ragged. Normally this happens because the text was copied and pasted and contains “hard returns” or tabs in the text that you can’t actually see. If this is happening to you, you can fix it manually by deleting those extra spaces:

If the alignment isn’t quite right in your Text elements (see red arrows), you can usually fix it manually just by deleting the extra spaces.

Note: If you’re copying and pasting text into your text elements and it all looks different, you will need to remove the formatting.


5. Font size

If you have really good eyesight, take a moment to pat yourself on the back, and then immediately go and check the font sizes on your website. Something that looks totally normal to you could have many readers squinting and scratching their heads.

A typical rule of thumb you’ll see on the web is to keep your body text at least 16pt. That’s a good place to start, but keep in mind that that number is entirely dependent on what font you’re using.

Fonts BenchNine, on the left, and Ubuntu, on the right, are both shown here at 16pt, but with dramatically different results.

As the examples above show, there’s no substitute for looking carefully at your own text and judging for yourself. Better yet, get a few friends to help you with some user testing.


6. Font Style

With over 600 Google Web Fonts to choose from, how do you choose the right kind of font? Here’s a basic recipe:

  • Headings: choose a stylized “fun” font. Since headings are large, short, and used sparingly, you can get away with a font that’s a little harder to read, but that adds visual interest.
  • Paragraph/Body Text: choose a plain vanilla sans serif font. These are designed to be read on a screen so they will provide a smooth, easy reading experience in paragraphs and at smaller sizes. Good options are Open Sans, Roboto, Ubuntu, Lato, and Noto Sans (the font used in these examples).
  • Navigation Menu: You can use one of your first two fonts here, or choose a third that complements the other two. Navigation menus have to be easy to read, so choose something with personality, but steer clear of cursive or decorative fonts.

Steer clear of all capital letters if you are writing more than a few words. Capital letters are great for navigation bars and short headings but are difficult to read in sentences.

Check out our blog post on Google web fonts for more recommendations.


Conclusion: your readability checklist

You put a lot of thought into your website text—and once you’re done writing the content, the next step is to make sure you’re presenting it in the clearest way possible. To review, here’s a checklist of readability factors to consider as you build out your website:

  • Line length: Use columns and images to keep your lines of text on your landing pages to fewer than 70 characters long. Blog post text can be somewhat longer.
  • Contrast: Make sure there is enough difference between your background and text color.
  • White space: Use it! Short paragraphs, bulleted lists, and Spacing elements are a good place to start.
  • Alignment: Use left aligned text and take the time to neaten up the right hand edge of your paragraphs.
  • Font size: Not all fonts are sized the same, so even if you’re using 16pt for your paragraph text, it could still be too small. Check with someone who doesn’t have 20/20 vision to make sure it’s comfortable to read.
  • Font style: When in doubt, use a Sans Serif font for your paragraph text. They are designed to work well on-screen. If you don’t want to use black, try navy, brown, gray, or another dark color.

Follow these tips and you can create appealing, inviting, and easy-to-read website in no time, which will let your actual message shine through.

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