Forth Bridge Letter

Forth Bridge Letter

This month I depart from the use of a selected quotation from a published work to publishing a Private Communication.

At the SCANZ Conference in Christchurch I presented a paper on "The History of Coatings in Harsh Environments" To illustrate this history I featured structures that had survived for many years, thanks to their protective coating, in harsh environments. I provided the history of the coatings applied to these structures. One of the structures I featured was the iconic Forth Railway Bridge constructed over the Firth of Forth in Scotland. This world famous steel structure was built between 1883 and 1890. It was the first substantial steel, as distinct from iron, structure built and the cantilever design made the Firth of Forth Bridge one of the strongest, and most expensive, bridges ever built. Information and pictures of the bridge were freely available on the net. All aspects of the construction, except the painting could be found. A reference buried in one of the documents found on the net enabled me to track down the paint company that supplied the paint for this bridge from it's construction until 1999. The company was Craig & Rose, a paint manufacturer since 1829. At the time it held the Forth Rail Bridge contract its premises were located in Leith near Edinburgh. The company still exists, now operating from new premises at Halbeath in Fife. It employs 75 people and produces 2.4 million litres of paint a year, specialising now in the manufacture and supply of high value retail decorative brands and traditional paints made to historic recipes. An e-mail to Craig & Rose resulted in the following communication from Colin Mitchell-Rose.

Forth Bridge Letter

Dear Peter

Thank you for your e-mail about the paint that was used for the Forth Bridge. When it was built, it was the first large-scale construction that used steel rather than cast iron. The paint system was particularly specified by the engineers and Craig & Rose's paints were only chosen after comparison with many other similar materials.

As soon as any part of the superstructure passed through the workshops or yards it received a good scraping down and 1 or 2 coats of hot boiled linseed oil (often they were dipped into heated tanks of the oil) followed by 1 coat of red lead primer (red lead and linseed oil). The inside of the tubes then had 2 coats of white lead paint. This has only required touching up once in 110 years despite seagulls and pigeons nesting inside them.

The externals got 2 coats of Forth Bridge Red Oxide of Iron paint. This was originally Finest quality Spanish Red Oxide (min 88%) with Boiled linseed oil and driers. The ratios were roughly 68% pigment, 28% oil and 4% driers and turpentine. This gave a very tough, flexible and durable coating which gradually chalked away. However flakes from the bridge still clearly show well over 30 distinct paint layers.

This formula was used successfully till after WW2 when experiments were tried with long oil alkyds and MIO as a substitute for Red Oxide. None of these were really successful, but in the mid 1970s, British Rail decided to change the binder to a styrenated alkyd. This had been used successfully on rolling stock and was faster drying and tougher. The pigment was unchanged but volume solids was reduced from 95% to nearer 65%.

This paint was used until the mid 1990s when neglect by British Rail and its privatised successors lead to some large scale flaking and failure of the system. After an outcry both locally and in Parliament (even the Prince of Wales got involved) new contractors were appointed and the decision taken to blast off all the old coatings back to bare metal and apply a modern 2 pack system using glass flakes such as are used in tank coatings and oil rigs. This work is now proceeding slowly and should be completed within the 20 year life span of the new coating system! What they will do then is anyone's guess - but the present lot will all be retired by then.

My personal view, now that we are no longer directly involved with its painting, is that two factors contributed to the crisis with the maintenance and appearance of the bridge in the late 1990s. Firstly, there was a serious decline in the actual painting done on the bridge. In 1973 when I started work here, the bridge took 400 gallons of paint per month. By the late 1980s and early 1990s they took that volume every year. Apart from a squeeze on finances and a reduction in the number of painters (as opposed to safety men, lookouts and admin staff), changes in legislation made access to the bridge very difficult. No longer could painters climb over the superstructure. Everything had to be done from scaffolding, gantries or platforms, and a glance at the bridge shows how complicated a structure it is. The collapse of the Tay Bridge in 1879 ensured that the Forth Bridge was well over-engineered with braces and cross-braces everywhere and even lowering a platform down one of the main tubes was fraught with complications. An 8 hour shift could actually produce only 1 hour of productive painting. And then there is always the weather to consider. A breeze at sea level could be a half gale on top of the bridge and Scotland is not known for its balmy weather.

The other factor, which has never been officially mentioned was the switch of binder from linseed oils to a styrenated alkyd. This switch was decided by British Rail laboratories in Crewe. My personal belief is that as this paint dried, it set up tensions in the lower films of older paints. Whilst these layers were generally adhering well with good inter-coat adhesion, there would inevitably have been places where because of surface contamination, it was not so good. In particular when looking at flakes in cross section, two black layers can be seen. These coincide with the two world wars. Rosyth, a major naval base, is just upstream of the bridge and during the war there was heavy traffic of coal or oil burning ships under the bridge. As little painting was carried out during the wars, the sooty layer built up and must be a potential fault line in the system. As long as the original slow drying paint was used, the potential problems were delayed. However once the faster drying system started to be used, surface tensions within the film must have started to increase.

With kind regards,
Colin Mitchell-Rose