From Mercedario Base Camp (Pirca de Polacos) Trekking Poles x2 a must.
Head west towards Mercedario over the rocky river bed aiming for the large yellow scree slopes. When you are within about 300m of the first scree slope head towards the bottom left corner where an obvious trail leads almost straight up the slope. Follow this trail as it winds its way along the ridges, and you will eventually be ejected into a small valley with the large scree slopes to your east.
Continue through this valley uphill for approx 1km where the path meets a small creek / glacial run off. Track up the right hand side of this creek for about 150m and cross over to the left (west) and a noteable path leads to the west up and over more scree.
After about 300m or so you will reach come across some large penitentes and exposed glacial ice, there are a series of naturally formed water ducts in the ice here, continue to the west following along side the water until possible to obtain the ridge.
From here its pretty much your choice of the easiest place to gain the "plateau" at the base of the south face, the scree here can be tricky in places as its quite shallow and sitting atop the hard ice on steep angles. Do not be surprised whilst climbing this if the whole side of a scree hill slips away.
Once on the "plateau" find an area of high ground as this whole area floods during the afternoon, there is plenty of crevasse but they are mostly chock full of scree and pose no threat.
An alternate would be to make camp on the ice/snow whicheveer your preference. There are (were) a few small crevasse around the base so dont assume its safe and just go walk-about.
The route from C1 to C2 takes between 12 and 18 hours depending upon your strength / whether etc.
From Camp 1 head towards the gap between the 1st and 2nd rock band (from right to left). The 300 - 400m to the Rimaya crevasse is quite deceptive, and feels a lot further than it is, again be mindfull of small crevasse within this area.
Cross the rimaya in any suitable place, as at Dec 2004 the crevasse was between 1 and 2m wide with the upper lip about 1m higher than the lower. Once the rimaya is breeched, stick to the right hand side of the 2nd rock band (good for resting).
From here on the face is between 40 and 50 degrees of snow with patches of ice. Pass the rock band and aim for the rocks visible about a further 300m or so above, the rocks are on a small rib which runs almost the entire length of the face. Follow this rib passing over some sections of ice where a belay may be required, when you have reached about 75% the way up begin to climb out onto the face proper, do not be tempted to continue directly to the rock towers above.
You need to aim for the top right hand (north east) corner of the face to enable an "easy" and safe exit. The last 100m or so is upto 65degree ice and deep snow. As you exit the face you will arrive at "la mesada" or camp 2 at 6000m.
From camp 2 you will cross the upper Caballito Glacier, avoiding the seracs by swinging past them to the North east from here its up over easy snow / ice (with the occasional crevasse for good measure) until reaching a prominent ridge. Several false summits and many hours later you will arrive at the true summit, directly exposed to the sheer West Face. Descent is via the same route.
It could be possible to descend via the normal route if required, you would then be forced to descend all the way to Guanaquitos (normal route base camp) and ascend the north route of Cerro Negro (non technical, and descend the normal route (southern side) / cerro negro south face descent route (non technical) back to Pirca de Polacos. Its possible, but a long walk.
Usual climbing boots / crampons
Technical Axes X 2
Ice screws 2 -4
Snow stakes 2 or more (for descent)
Stove & Pot for making water (its a long day)
Food / Tent etc