Lockpicks[]
These are essential for the thief to use this skill at anything like his "natural" ability level. It is these items which are listed in the Player's Handbook as Thieves' Picks, costing 30 gp. They are usually short lengths of rigid wire and flat, narrow plates of iron or steel, especially designed and made for dealing with locks, and there will be a dozen or so to a set. They will be supplied either on a ring (like keys) or in a cloth or leather wallet which unrolls. These items will never be freely available on the open market.
Improvised Lockpicks[]
Lockpicks are made for the job, but it is possible for a resourceful thief to improvise a lockpick from a length of wire, a hairpin or hat pin, or some similar ready-to-hand item. Obviously, this will never be anywhere as good as the specially-crafted item, but it's better than nothing. As stated in the Player's Handbook, a penalty of anywhere from -5 to -60 can be applied to the use of such improvised lockpicks. The following suggestions are given for the DM to select within this range.
To create improvised lockpicks the thief must make an Intelligence check, modified (positively!) by his experience level—reflecting the resourcefulness greater experience brings. If this check is successful, the improvised items can be used with a penalty of -5 to -30 (1d6x5) to the open locks roll. If the modified Intelligence check is failed, the thief can bodge up something, but it is a feeble effort—the penalty is -35 to -60 (1d6x5, +30) for the Open Locks attempt. Locks which are of Masterful quality, however, cannot be opened with improvised lockpicks (unless the DM allows the 01=success rule here).
Critical Failure: When opening locks, either with lock picks or with improvised lock picks, the DM may rule that on a skill check roll of 96-99 the tool snaps but can be wangled out of the lock if a Dexterity check is made; on a roll of 00 the snapped tool is wedged in the lock and cannot be removed (save for disassembling the whole lock!).
Special Rule, Combination Locks: The DM may wish to allow the existence of these complex locks in his campaign world. If he does, then such locks are always of Superior or Masterful quality. As an optional rule, a thief who makes a successful detect noise roll when trying to open such locks can halve the usual negative modifier to his skill for such formidable locks, as he is able to detect the sound of tumblers falling!
Acid[]
If a thief is faced with a lock which his best efforts cannot pick open, metal-eating acid is one alternative. Such acids will eat through locks if the locks fail a saving throw (for metal) vs. acid (this save is 13). If the save is made, the lock cannot be opened, but it will be ruined (and unopenable!) if a second save (metal vs. acid) is failed. Use of such acid is difficult and avoided by most thieves, for various reasons.
Use of metal-eating acids is difficult because only acids of great strength will do the job effectively. The DM should greatly restrict the availability of such acid; acids of the strength of black dragon acid and thessalhydra acid (possibly also giant slug spittle) are among the few known effective metal-eaters. Thus, availability is very low (and cost very high).
Thieves usually avoid such acids in any event. First, the acid is very hazardous to carry. While it may be contained in glass containers (and possibly ceramic), such vessels are fragile. Imagine falling down a pit and hearing the sound of breaking glass as double-strength acid begins to seep through clothing and over one's back . . .
Second, if the acid does not do the job it may ruin the lock and any hope of opening it in another way, as described. Third, it is a calumny on the professional reputation of a thief to have to resort to such means as acids!
Chisels[]
A thief may attempt to force a lock open with a lock chisel and a small hammer. This is not really a highly skilled activity, and the DM might consider extending this to non-thieves. The base chance for success is equal to the open doors percentage (which is Strength-based, of course). A thief may add one-fifth of his open locks chance to this base chance—knowing something about locks does give a slight advantage here. Obviously, forcing a lock is a noisy activity and any hope of subtlety and surprise evaporates with the first blow.
Cutters, Files and Hacksaws[]
These instruments may have to be resorted to if a thief cannot pick a lock, but thinks he has the time to try these desperately slow methods for bypassing the lock. With a file or hacksaw blade the thief can try to saw through the lock apparatus; a pair of small wire cutters may also be useful for disabling some part of the internal mechanism. Usually, only reasonably small locks can be cut through in this way. It may also be possible for the thief to cut around a lock with a hacksaw blade.
Again, use of such instruments is often fairly noisy (although nowhere near as noisy as using a lock chisel). The main drawback to cutting through or around locks is that it takes a very, very long time. in most cases the attempt will be certain to succeed, unless there are special circumstances—e.g., the thief has only one small rusty file and the lock is a huge combination lock device!
Magnifying Glass/Lens[]
At the DM's discretion, using such an item to inspect a lock may add 5% to a thief's chance to open the lock. This only applies if the thief has some element of the lock apparatus exposed to his view—if he can see something of the inner workings of the lock.
Oil and Funnel[]
Some locks may grow rusty with age and disuse, and be harder to pick than new locks of the same quality of make would be. Seasoned thieves know only too well that the locks of doors in dungeons are all too often of this sort. The DM may apply modifiers (-5,-10, etc.) to the open locks chance for such doors as he sees fit. rusted and even fairly dirty locks may have a little light oil applied to their insides, usually with the use of a long-necked funnel (and maybe with a short length of rubbery tubing on the end of that). The negative modifier may itself be negated, in whole or in part, by such lubrication. It takes 1 round to apply the oil and 5-10 (d6+4) rounds for the oil to have its effect on the mechanism.
Of course, using oil to lubricate a lock helps the thief to open it silently. If the DM wants to make a silent movement check for lock-picking, it should be made at +10 if oil is used. Oil is also useful when it comes to rusty door hinges in this context.
Time Scales[]
Picking a lock takes 1d10 rounds, as stated in the Player's Handbook. If the thief has no lockpicks or is using improvised lockpicks, this duration becomes 1d20 rounds.
Other ways of bypassing locks take rather longer. Using a lock chisel and hammer takes one turn; metal-eating acid takes 3d6 rounds; cutting around a lock takes 2d6 turns (assuming that the door is a wooden one of average thickness); cutting through a lock takes 3d6 turns. The DM may roll randomly for time needed, or may select a value within the range shown which reflects the quality and size of the lock. Size will most affect acid and cutting round a lock, whereas quality will affect cutting through a lock, as regards time needed.