Lightfastness is a key issue for the serious artist. It is well known that, over a period of time, colours will fade or change, particularly when exposed to environmental factors such as light, heat, dust, humidity etc. That's why Derwent continually test the lightfast rating to give you the best possible performance for each colour.
The test we use at Derwent is the ISO 105 Blue Wool test, which was originally developed for the textile industry. A scale of 1-8 is obtained, with values of 6 or more being considered GOOD and will last in excess of 100 years in gallery conditions.
As well as lightfastness, we submit our pigments to a rigorous colour test using a spectrophotometer, which is an instrument that measures the amount of photons (the intensity of light) absorbed.
The colour test measures colour difference and comprises the following indices:
- Lightness or darkness
Using a Delta E measurement (how the human eye perceives colour difference) we compare batches with a standard; a score <3 being considered acceptable. Colours are adjusted using this machine and also using the human eye by our highly specialised team.
Every batch of pencils is manually tested for the appropriate texture (softness, waxiness, scratchiness) and, in the case of water-soluble products, its solubility. We test all pencils against a standard for texture.
In the near future we are looking at implementing a further test using a writing machine that will repeatedly use a pencil to test its texture and degree.
A key test is the tensile strength of a pencil core. There are two machines that test this:
- Hand-operated which measure in Kg and newtons
- A tensiometer which measures the tensile strenght of a material. We break points with this machine. This can also be used to measure the breaking point of a pencil held at an angle, the strength of a rubber at the end of a pencil, the tensile strength of the wooden barrel and bonding in the glue.
We break the pencil strips before they are put in wood and ensure they are strong enough.