My characters always arrive late to the scene, so they find blood. What I want to know is what is a realistic time frame for blood to dry so I can estimate how late they are to the scene.
Pool and smears. Usually a combination of both. How about if it was in the shade/in the sun? And how long till it congeals from the time of it being fresh?
Sealed tubes of blood (rubber stoppers) don't dry out for weeks. But exposed to the air blood will dry very quickly. There are lots of web pages on forensic science that can best answer your questions @yanlins.
there are too many variables for anyone here to give you a definitive answer... things like temperature and relative humidity in the area, whether there is a flow of fresh air, or if it's a closed space, whether the person who bled was on blood-thinners, the surface the blood rests on, and a slew of other factors will all affect the timing... you need to do some creative googling...
Even a large quantity of blood spilt on the floor will quickly spread out to form a fairly thin layer with a large surface area. Perfect for evaporation. From experience, several litres spilt on the floor early in the morning will still be quite tacky into the afternoon. To completely dry would be overnight at least. A thin smear on a microscope slide will air dry in about 20 minutes ready for staining with Papanicolau stain.
I've never heard of using a pap stain on blood. Is it something they do with animal blood or something?
Blood will take ten minutes more to dry than it will take for a suspicious gendarme to notice it dripping.
Got confused with semen morphology staining. It was late at night! I was thinking of Giemsa's stain for differentiating leukocytes.
"A single drop of blood that falls on a dry surface often dries in about an hour at room temperature. Blood that has collected in a pool dries slowly. The drying time depends on the size and depth of the pool. Temperature and humidity also affect the drying time." More here.
An hour for one drop! Try minutes at most. That source is unreliable. I can tell you from experience it takes very little time for a single drop of blood to dry. Not only that, a source that doesn't specify what is meant by a "drop" is problematic. Drops can vary in size depending on what they drip from and how fast they dry also depends on how porous the surface is the drop falls on.
I fortuitously nicked my finger this morning, so I put a generous drop of blood on the bottom of a ceramic coffee mug. The drop measured about 1 cm in diameter. The indoor temperature is about 70 F and the humidity is about 60%. I placed the cup where it wouldn't be in any drafts from heat vents. After one hour, the the drop had formed a dried rim about 2-3 mm wide. Inside that rim, the blood had formed a surface "skin." I broke that with a toothpick and found that the blood underneath was still liquid. So it appears that the source I linked to earlier is correct in saying it takes about an hour for a drop of blood to dry at room temperature. Were the humidity lower and temperature higher than in my experiment, it would probably dry faster. For purposes of fiction writing, you could plausibly arrange circumstances (e.g., a cool, damp basement) so that a larger quantity of blood wouldn't dry for several hours, or (e.g., outside on a breezy spring day in Arizona) so it would dry rather quickly.
A centimeter? That's a lot more than a drop. I don't doubt that a small blob of blood on a non-porous surface would take that long to dry. That's the problem with these cited times from the web pages. Hematocrit (thus viscosity), clotting factors (lots of reasons those can vary), the surface the blood is on, the atmosphere including temperature and humidity, and the shape of the blood (like thick or spread out) will all affect the time it takes to dry. Best to consult a forensics science source.
