Making Spherical Mirrors: Part 2

Making Spherical Mirrors; Part2. Using Modern Materials

This post is about how to construct a 9 piece frame for a segment of a spherical mirror. For the sample frame I used 30cm x 30cm square mirrors from IKEA.
 

Spherical mirrors sets are part of a ball. Imagine you have an orange and you cut a small square out of it's skin: This small 'spherical' square is what we are trying to create. When flat plane mirrors are placed into this arrangement, it will form a segment of a sphere: See part 1 for an explanation of the maths and what's being done.

At first, I tried using shaped plywood and a 16 mirror frame: 

Making Spherical Mirrors: Part 1

Introduction: Part 1

This post is about how to construct simple spherical mirror frames. For the sample frame (to be described in part 2) I used 30cm x 30cm square mirrors from IKEA. I'll show how it works here and then describe in part 2 how to make this even more cheaply.
 

Spherical mirrors sets are part of a ball. Imagine you have an orange and you cut a small square out of it's skin: This small 'spherical' square is what we are trying to create using mirrors. However, we are making it out of flat pieces of mirror. 

Imagine the square of orange peel has one square mirror placed at it's centre. When eight other mirrors are placed around the centre one, each edge (away from the centre) will be raised slightly: It will follow the curve of the ball. 

Sunrise from a Neolithic barrow

Sunrise from a Neolithic barrow near Beachy Head, East Sussex

Just became the expert on Simon Mayo's Drivetime with the question 'Can the naked eye see the curvature of the Earth?' Photo above is of the experiment I described on the Neolithic mound near Beachy Head (which happens to be one of the few places you can do the experiment).

Simon also asked if you could see the curvature of the earth somewhere like Cornwall. I said no, not directly, but didn't explain why: The reason is that the only time you can see a far horizon is at sunset and sunrise (possibly moon-set & rise also). The rest of the time, the real far horizon blends in with the sky. This optical illusion (of a low horizon) is generated by haze within the atmosphere.
 

Neolithic tin mirrors: Polishing experiments

This post describes a tin polishing experiment using only those materials known to exist in late neolithic Britain (just after the discovery of tin).

After casting of a tin mirror, a small slab of tin if left. At this stage, the surface does not resemble a mirror.

Stage 1:

Use a piece of hard beach rock to break up a piece of chalk:

Stage 2:

Crush the chalk using stones or a pestle an mortar, then add fat or oil and mix to form a white clay.

Stage 3:

Polish the tin plate using a rag or dried leaves: The clay will turn black as the polishing progresses. This process takes a long time!

Stage 4: 

When complete, the polished area will take on a high lustre:

Summary:

Materials needed: Chalk, fat or oil, a hard rounded stone, a rounded bowl-stone (pestle) and cloth or animal hide:

With these materials, it is relatively simple to produce a good quality tin mirror.

Neolithic tin mirrors: Casting experiments

After smelting from its ore (cassiterite),  tin is a soft metal which melts at 232 degrees centigrade.

I tried several methods to make a casting for a tin mirror using only those materials known to exist in late neolithic Britain (just after the discovery of tin). Of the methods tested, the following process seems to produce the best results:

Do not try this at home without suitable safety equipment.

Stage 1:
Use a flat bottomed pottery dish placed level within hot embers. Carefully add filings of tin:

The tin will gradually melt into the bowl to form the initial shape. Because of surface tension, the shape won't form unless you have about 3-4mm depth of tin. After the tin forms a flat surface, pick out any surface impurities (using a sharp piece of stone) and then leave to cool.

Stage 2:
Turn the cooled tin plate over onto a polished flat surface, preferably something which will cool slowly (such as polished slate, pottery or obsidian). Re-heat the tin and then leave to cool again:

In the example above, we've turned the tin onto a baking tray and re-heated. As can be seen, if the tin is allowed to cool too quickly, deep ridges will form in the upper surface (this may not matter if you only need a one-sided mirror).

The casting is now complete. The next stage is polishing (see link here).