There are really two aspects to this question that need to be answered.
What will be the central theme?
How broad should the collection be?
In my case choosing the central theme was easy - drawing instruments. I also immediately decided that I would like to collect a reasonable selection of slide rules but that this would always be secondary to collecting drawing instruments. More recently I have added other types of instrument to the collection. However, this section is mainly written around my collecting drawing instruments.
Deciding the breadth of the collection was more difficult and this has changed with time. Drawing instruments have for centuries been used for navigation and I therefore widened the scope to include some navigational instruments where these were related to drawing instruments. On this basis I would add Capt. Field's pattern parallel rules and a station pointer. In the related field of calculation I would also include pocketable mechanical calculators, such as Fowler calculators since they were contemporary alternatives to slide rules. I have therefore chosen to collect over quite a broad field.
Period is another aspect of breadth. Initially I had not intended to collect modern instruments (i.e. less than 40 years old). However I already had some (from my days as a professional engineer) and others began to come my way so I decided that I would add these to bring the collection up to the present day. How far back in time to go will largely be determined by what one can afford - in my case I decided that I would try to get back 250 years to the mid eighteenth century and I have managed to find some affordable items.
My collection is now quite large comprising about 300 cases containing sets of instruments or specialised single items. It is still growing so one may wish to place more constraints on the collection, such as collecting the instruments of one manufacturer only, or those from a particular period such as the late nineteenth century.
I started by looking in local antique shops (more like junk shops in many cases) and antiques markets and this is still a useful source for both sets and individual instruments. One soon gets to know the shops that have instruments - generally those that specialise in collectables, nautical items, scientific instruments in general, or old tools. If you can develop a relationship with the proprietors of one or two this can be useful as they will look out for items for you.
General antiques fairs are another source, especially the larger ones where there may be specialist dealers.
The rarer items may be found at the Antique Scientific and Medical Instrument Fair, held each April and October in central London. See www.scientificfair.co.uk. There are usually over 50 dealers including several from the Continent and the USA. However you may have to pay "London" prices.
Auctions are another good source. Bonhams and Charles Miller hold specialist scientific instrument auctions but these usually only include rare and top quality items which will be fairly pricey.
Internet auctions provide a good selection, especially www.ebay.co.uk,which I can recommend.
1. Fakes and reproductions:- fortunately fake drawing instruments are very rare but other types of popular scientific instrument, such as sextants and magnetic compasses, have been reproduced in large numbers in India. Often they are stamped "Stanley London", which is now the registered trading name of one of the importers. Close examination soon shows that they are of grossly inferior quality and the wooden box is often a giveaway - the wood is not right and the brass banding is not inset.
2. Made up sets:- Closely examine the instruments in a set to ensure that they match in the fine detail. Check the maker's name on each instrument. Check that each item properly fits its place in the case.
3. Seriously incomplete sets:- It is not easy to find missing items, particularly for the older British sets. I've spent a lot of time trying with very limited success. Pens and rules are the only things you are likely to find at all easily. Thornton Minerva and Techset instruments are also fairly easily matched as they were mass produced in very large numbers.
4. Highly polished instruments that should be lacquered or blackened. One frequently finds ex WD rolling parallel rules which have had their black finish removed and have been highly polished.
Half the fun of collecting comes from researching the items once you have obtained them. Start by reading the books listed in the "References" section; this will give you a broad appreciation of the field. Next, visit one of the museums that specialise in this field, such as the Science Museum in London. There is no substitute for viewing actual items.
Try to obtain photocopies of makers' catalogues or better still actual catalogues if you can find and afford them. They were often 100 pages or more in size and give a real insight into the vast range of instruments and sets that used to be available, for instance, 100 years ago.
Auction catalogues can be quite useful as well - they can be used as rough price guides.
Dating instruments can be particularly difficult. Often the same basic pattern was made for 50 or even 100 years. Fortunately ex Government instruments are likely to be stamped with their date of manufacture. Slide rules by some makers (e.g. Faber Castell and Aristo) have date codes stamped into them and others (e.g. K&E, Otis King) have serial numbers from which the rule can be dated to within a year or two. Care is required as the same serial numbers were often used more than once. Other instruments may have a date on a calibration certificate or there may be a bill of sale. The construction of a box may be another clue - for instance, comb jointing was a late nineteenth century invention. Other clues are in the address of the maker or retailer if it is printed on one of the instruments, the case or instructions - some of the references I have quoted have detailed information on makers with dates for when they operated from their various addresses Patents and patent dates may also be given on instruments but they only indicate the earliest possible date as they often continued to be quoted for many years after the patent had expired. Contemporary catalogues are also a help, as are dates given on various Internet sites, although the latter are occasionally wildly wrong. Unless I have quoted an exact date or a specific range, dates I have quoted should be considered as very approximate; they are merely my best estimate based on the information that I have to hand at the time.
It can also be helpful to join one or more of the specialist societies, such as the Scientific Instrument Society or the Oughtred Society. I have given links to these and others on my Contacts and Links page.
Caring for the collection
Much has been written on the care of old instruments and I do not intend to repeat all of it here; instead I will concentrate on things specific to mathematical instruments and consider in turn each of the things which can damage them.
1. Heat - this mainly affects the wooden cases and causes them to dry out. Eventually the wood may split or joints fail. It also causes plastics to distort and deteriorate more quickly. Leather also dries out - I use old fashioned shoe polish of the correct shade to prevent this. Wooden boxes are best polished with beeswax.
2. Light - this is particularly harmful to fabrics which will fade and become fragile. It will also affect some plastics, causing them to yellow or become brittle. If boxes are displayed open which have fabric linings then they should be kept out of bright light and especially sunlight. Sun light also affects ivory but sometimes can be used to good effect as it will bleach badly yellowed ivory.
3. Damp - this causes many problems including corrosion of metals, staining & rot of wood, staining and rot of fabrics and marking of paper. It will also dissolve some glues.
4. Incompatibility - "Celluloid", chemically cellulose nitrate, and cellulose acetate slowly decompose giving off an acidic vapour which combines with the moisture in the atmosphere to form acids which will corrode any metal in the vicinity. Unfortunately celluloid was used for many parts of instruments until it was replaced by more modern plastics, such as acrylic in the 1960s. Keep transparent celluloid items well away from other items in the collection. White celluloid is much more stable than the transparent kind.
5. Insect attack - woodworm and the clothes moth are two well-known attackers. Other insects eat paper and even glue. One technique for dealing with these is to place the item in a freezer for a few weeks.
6. Fungi & moulds
7. Careless cleaning - many of the larger items in brass originally had a lacquered finish and it is important to preserve this; therefore the use of abrasives and metal polish on these items should be avoided. Too frequent use of metal polish will eventually round corners or obliterate signatures on any metal part. Metal polish will also stain the fabric lining of instrument cases. Cleaning solvents will remove wax polish and may also damage plastics. Cleaning of slide rules is well covered in The Oughtred Society Slide Rule Reference Manual.
The important thing is to regularly inspect every item in your collection for any problem signs. Only by doing this can you ensure that a serious problem will not occur.
It is also a good idea to keep a detailed catalogue of your collection and to photograph it. I keep a catalogue in a book and also on the computer. I label each set or item with a tie on label (never a sticky one). I keep detailed records of any additions I make to sets. I also note details of any restoration work I have carried out, and also record the details of any research into the item or its provenance.