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SCIENCE

Drying Times

Curriculum links

English National Curriculum
Science 1: 1b, 2d, 2h & 3: 1a

Approximate time required: 60 minutes

Resources needed

Per group:

1 Strip (approx. 15 cm x 5 cm) of each of:
- Card
- Paper
- Plastic (e.g. bag)
School paint, sufficient to coat each strip
Paint brush
Stop clock, stop watch, or 5 min sand timer
Knitting needle or crochet hook

Suggested organisation

Children work in small groups, with initial and final class discussion.

Carrying out the activity

The class are told how industry tests the drying times of the paints they develop. Drying times are important, as customers often demand minimum drying times. Industry uses a simple machine which slowly drags a lightly balanced pin through a sample of paint. Eventually the pin will no longer leave a mark in the paint, as it will have dried. As the pin is dragged through at a constant speed, the length of the mark made by the pin is an indication of the drying time of the paint.

Children rarely appreciate that scientific testing is carried out on industrial sites, so after gathering their ideas, the teacher should point out that most industrial sites have laboratories, which test the quality of their product, and carry out tests to try and improve their products - in this case, coloured pigments to go in paints.

Children are now asked to think of ways in which they might mimic the testing process in the classroom. They are challenged to devise a test which will test the drying time of school paint on three different materials - plastic, card, and paper.

If they find it difficult to think of their own ideas, they can be shown a selection of the resources listed above. Any children who still have difficulty in devising their own test can be given the idea of dragging a knitting needle for about a centimetre through the painted strip, and repeating this every 5 minutes until no mark is left.

They then need to think about fair testing, measuring and recording their test, for example:

• use a timer to measure 5 minute intervals
• perhaps attach a mass near the point of the knitting needle, to provide an even force to the 'drag' each time
• if using powder paints, children will need to ensure that either enough paint is mixed to paint all three materials, or that the same quantity of water and paint are mixed each time (they should keep a record of these quantities in order to reproduce the mix)
• recording - the strips will provide a permanent record - the number of marks showing the number of 5 minute intervals which have passed
• children can produce a simple table showing the drying times for the three materials or one can be prepared for less able children
• less able children may also be expected to record the number of marks in each strip, rather than the number of minutes per strip.

Children carry out the tests they have devised. They can then be asked to present a brief 'Laboratory Report' on their tests, explaining how it was carried out, what they measured and how they made it a fair test.

Extensions / links

Art
Make use of the fact that paints need to dry in order not to 'bleed' by painting a background using paints adjacent to each other while wet, deliberately, allowing the colours to bleed and merge. When that has dried, paint an image over the top, using thicker paint and being very careful not to allow it to bleed or merge. Experiment further and use this technique in more artwork.

Art
Use the technique of dragging wet paint with a piece of card cut with a serrated edge to produce designs in straight or wavy lines. Work quickly and don't paint too large an area at a time to work on. Wood-grain effects can be achieved by this method. It may be helpful to add some paste to the paint so that it drags more effectively.

Science
Other examples of testing techniques in an industrial laboratory are shown in the activity Quality Testing of Dyes.