Thanks for the back up folks: having had to make up countless baths of caustic soda over many years - as part of an anodising process production line - take it from me that our buyers would not have let us buy 10 gallon drums of ready made up 50% caustic if there had not been a regular problem of charge hands trying to pour pellets into water much too quickly and blocking up the whole system for hours while we then tried to dissolve the resulting 'caustic berg' and having to replace whole sections of pipe work that water would never reach again(Occasionally they would pour in 10 gal drums of conc hydrochloric too - or even bleach - which can be really exciting!). Indeed, the reason that caustic is generally available industrially as 'pearl' - granules the size of split peas - is to take advantage of the reduced ability of the lower surface area thus presented to take up water from the air, as would happen much more rapidly with powder. Why do so many people out there immediately jump to call others liar?
Furthermore, those thinking of using this stuff in enamel and even glassware, might like to note that it will gradually dissolve even these - which is why when you see caustic solution bottles in labs they are nearly always cloudy: and why you shouldn't put decent glasses in the dishwasher... Those OH- ions are gonna do their stuff whatever the chemical you used to get them into solution (Which is why your caustic phosphate containing washing powders tell you not to put them in the bath.).
I might add that as well as in the lab, I have had on a number of occasions had to risk getting burned trying to remove solidified caustic soda from drains, after people just dumped it in without dissolving it first. They are not entirely foolish in using the granules directly as, if you can coat say a lump of hair with the powder it will quickly burn it away (hence don't try applying it with a natural brush!), and it is only natural not to want to pour the stuff away before it has had time to work: but for completely blocked systems, unless you have a constant replenishment of water over some time, the water immediately in contact with the caustic rapidly saturates and you are then stuck and have to call me out with my long hose to put a jet of water where it is needed.
Washing soda, on the other hand, naturally has an enormous amount of 'water of crystalisation', but is sold in a low state of hydration (cos we don't want to be buying water unnecessarily do we?). This means that once you have opened the bag, you better use it quick, cos next time you look it will be just one big lump - having absorbed loads of water from the air - and you'll need a hammer to deal with it. This is a shame because I consider this, along with caustic, citric acid (available in decent sized bags from Asian food shops I find) and bicarb, meths and white spirit, to be some of the 'must have' essential chemicals for around the home (10% citric with a dash of washing up liquid seems to clean just about anything round the kitchen and bathroom, I find.)
Cheers,
S