A descriptive title is a rare thing, but in Commemorative Glass, Nick Crowe sticks fast to his chosen medium, focusing his efforts firmly on ideas of remembrance, and the making of monuments. This does not mean, though, that Crowe has crafted a consistent show - it is, if not quite incoherent, then erratic, with sudden shifts in tone and a sophomoric approach to politics that, together, serve to undermine his work.

Operation Telic is a lengthy series of drawings engraved on to glass, mounted on shelves and illuminated with a sickly green glow. The images are copied from the Ministry of Defence website and, given this source, do not dwell on the horrors of war. Instead, smiling children high-five troops and soldiers are shown passing out rations to grateful crowds, in earnest conversation with Iraqi citizens or bloodlessly patrolling empty streets. Crowe might have left it at that - his use of a material associated with clarity and transparency to enshrine these propagandising images is subtle and articulate - but he takes a step too far.

The shelves that bear the glass plates are slathered in black goo, a crude reference to oil, and the wiring for the lighting system hangs low, hinting at the threat of Improvised Explosive Devices. Yes, Nick, we get it - the jolly soldiers often end up dead, and the invasion of Iraq wasn't all about regime change.

A second major work, The Beheaded, is problematic, too. It is a beautiful, mesmerising piece that memorialises all those who have been executed by beheading in the first five years of this century. Headless torsos hang from a rotating frame, cut from sheets of dichroic glass, which simultaneously reflect two colours. The result is a shimmering rainbow of projected bodies, a simple collision between an emblem of tolerance and the sorry results of intolerance. But what exactly is Crowe trying to say here, turning 68 violent deaths into an outsize equivalent of a child's mobile? He might be making a point about the potential for spectacle as a method of social control, or hinting that the bare facts of barbarism require sugar-coating and, literally in this case, spin, but the more one investigates the piece, the less it seems to mean.

Next comes a seismic shift in tone, with The Family Tree Of Zainab Duranthrrial Sadik Llthnnnzstol, the First Martian Martyr. Commemorating a fictional political activist murdered on Mars in the year 2226, the titular family tree reveals that Ms Llthnnnzstol is, or will be, a human-alien hybrid. The human half is, we are repeatedly told, Muslim. Get it? Crowe is using futuristic fantasy to remind us that we Westerners might have a few problems dealing with the other, the alien, on our own planet, today.

All this might not matter - sub-par artists mount dud exhibits all the time - but here disappointment sticks in the craw because, in among the unsubtle politics and unfortunate shifts in tone, Crowe shows that he is capable of rigorous, affecting work. The Campaign For Rural England is a witty appropriation of familiar street furniture, the bus shelter, its standard cold steel frame replaced with warm, hand-carved English oak, its glass panels shattered but intact.

The clash of an urban form with rural materials is neat, the frozen moment of unconsidered violence is powerful, and there is a more subtle nod to the causes of conflict in the transformation of transparent glass into an opaque divide. Similarly, Oh Christmas Tree! is deceptively simple, punning on the fact that The Red Flag shares a tune with a carol by crudely rendering a Tannenbaum in smashed glass fragments.

The pitching of revolutionary violence against jolly tradition is a simple tactic, but the piece raises more subtle questions about the peculiarly un-Christian practice of idolising a fir, or the irony of the anthem's continued use by a post-Socialist Labour party. More simplicity and subtlety comes in the form of an ambivalent panegyric to the internet. Proposal for the World Wide Web is a sketch by Tim Berners-Lee, elegantly outlining his world-changing invention, on paper and etched on to the glass of its frame, a quiet act of commemoration. The same treatment is used in June Becher, a memorial web page, and Computer Spy, a fake warning page - the former is a sad evocation of the quest for permanence amid constant virtual flux, the latter a hint at the paranoia and threat that lurks in the darker corners of the web.

There's a good show hidden behind Commemorative Glass, then. Crowe, when he wants to, makes layered, eloquent work that blends surface wit with deeper concerns. But the strength of his more considered work is eroded by bursts of glaring unsubtlety and an unfortunate tendency to undermine ideas by burying them beneath ham-fisted polemic.

  • Nick Crowe: Commemorative Glass is at the CCA, Glasgow, until March 31.