Workshop Practice, Tools, Equipment and Safety
This domain covers the dyehouse as a workplace: correct equipment for each job, accurate weighing and measurement, batch records, and safe handling of the corrosive, toxic and flammable chemicals involved. Questions are often practical - what vessel, what balance, what first aid.
Tools and equipment
- Weighing - a balance readable to 0.1 g for dyes and 1 g or better for salt and soda.
- Vessels - stainless steel for reactive and most dyebaths; iron and copper contaminate shades and catalyse dye decomposition.
- Measurement - pH meter or indicator papers, thermometer, measuring cylinders, pipettes, stopwatch.
- Machines - hank reel, package dyeing machine, jigger, winch, padding mangle, hydro extractor, dryer, stitching and cutting tools, sieves for dissolving dyes.
- Records - a batch card listing shade %, liquor ratio, temperature programme, chemical doses, pH and times, so a shade can be reproduced.
Safety rules
- Caustic soda: goggles, rubber gloves and apron; always add alkali to water, never water to alkali, because of violent spattering.
- Acids: add acid slowly to water, never the reverse; keep bottles below eye level.
- Sodium hydrosulphite: keep dry and cool; moisture and heat can cause spontaneous heating.
- Hydrogen peroxide and dichromate: oxidisers, toxic and irritant; store separately and avoid contact with reducing agents.
- Wax pots and boilers: fire risk; keep lids and extinguishers accessible.
- Splash on skin or eyes: flush with copious running water for at least 15 minutes, then get medical help.
- Effluent: neutralise and treat before discharge; dyehouse waste is alkaline and coloured.
Worked example
A 25 kg yarn lot needs a 2% shade. Dye = 0.02 × 25 000 g = 500 g. On a 1 g balance the weighing error is about 0.2%, acceptable. If the same lot were 200 g, dye needed is 4 g, so the same balance gives a 25% error - a 0.01 g balance is required. Reasoning: accuracy must be proportional to batch size.
Traps
- Stirring reactive dyebaths with an iron rod.
- Recording only the shade % and not the liquor ratio or pH.
- Adding water to concentrated caustic soda.