Clad in my rainbow lycra tights (well this is what my hero Andy Pollit wore back in the 80's) I set off up my first slate route, 'Seamstress' (HVS 5a), not sure what all the fuss was about. I had heard horror stories about slate being run out, loose and scary and this route was none of these but by my third route 'Pull my Daisy' (E2 5C) the rumours were definitely confirmed. It's not all bad but climbing on slate is a unique experience which requires certain skills and knowledge.
Slate is a smooth medium to climb on and making the most of friction is essential. Climbing in the summer heat with perspiration forcing its way out of your finger tips as you pull on small or smooth holds is unsettling. Hot rubber on the soles of your boots rolling off the holds is also unnerving. Spring, autumn and summer evenings (in the shade) are optimum times for grip. In addition to this, if the air or the crag is damp then go and climb somewhere else and never get caught out in the rain on a lead - you have been warned! Slate does dry quickly however after the rain has stopped.
Many of the classic climbs now have very little loose or fragile rock on them. The tops of the routes do however require caution whilst topping out, taking rope in and belaying your second. There is often a pile of slate waste to negotiate at the top of a climb that either you or the running rope might dislodge. Slate cleaves naturally into thin sheets and provides plenty of razor sharp edges that will cut a tensioned rope. The less traveled routes can offer loose, snappy, crumbly and dirty rock where care is needed and it is worth having a helmet with you - if only to wear when belaying at the foot of the route as your mate pulls over the finishing holds onto looser ground.
Most slate routes were part of working quarries as recently as 50 years ago so their stability is not guaranteed. There are lots of stories of slate routes moving or even falling down, so if the guide book you are using isn't new then look hard at the routes to make sure they are as described in the description.
To be honest, to enjoy the slate experience you need to be climbing VS 4c comfortably. Drop your grade to start with and build up your understanding of the rock before pushing it out. Read the route descriptions carefully to glean any specific information about gear, moves etc. Remember, many of the VS grade climbs were soloed by E6 leaders!
Your will find a variety of styles of protection on slate. If routes are protected with nuts and cams it will be because they follow crack lines. A variety of pieces will work in slate and often right down to the smallest of cams in parallel cracks and micro wires in tapers. Take plenty of RP type micro wires. The rule on slate wherever the rock allows, you should place as many runners as possible.
Many of the routes were equipped with bolts in the 80's. Back then many of the local activists were trad climbers with a reputation for bold ascents so be prepared for adventure. In addition to this, many of the original bolts were small 8mm stud types and are now rusty so best not fall off on these! Take plenty of small wires, RPs and on harder routes a few skyhooks.
The easiest way off is often back down the route but if you are abseiling off in-situ gear check it carefully. Don't lower off a route unless you know your rope is going to run free of all sharp edges.
Leaving your sloppy, worn out boots in your sac would be my advice. You need new boots that are a tight fit and have good edges, as climbing on slate requires you to stand on minute edges.
Due to the nature of slate the holds tend to be fewer, smaller and more spread out than other types of rock. This lends itself to a gymnastic style where flexibility and a bit of lateral thinking go a long way! Three common types of move are the 'rock over', 'mantelshelf' and 'crimp'. Precise foot placement and careful weight transfer will make all the difference so it is worth practicing these movement skills on the climbing wall on smallish edges.
Most slate climbing takes place in disused quarries. Quarries are hazardous places and owners are keen to keep most people out for their own safety. As climbers, our presence in quarries is generally tolerated or a blind eye is turned. Find out about the local access situation and keep a quiet presence. There are some exceptions where quarries have become country parks e.g. the Vivian in Llanberis or when owned by public access bodies.
Slate climbing in North Wales is under going a renaissance. To further promote this, local activists have undertaken extensive re bolting(Winter 2006/7). This has lead to many routes becoming a safer proposition. Further information can be found by googling 'safe slate'. Finally, to wet your appetite and see the slate master himself in action get a copy of the Stone Monkey video and watch Jonny Dawes ascent of 'The Quarryman'.