TL;DR
Hytale does not give you a placeable glass block right now. What players call glass comes in two practical forms:
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If you only need glass for a house, you want windows, not bottles.
What glass means in Hytale right now
If you came in expecting Minecraft-style sand-to-glass blocks, the game will feel confusing for about five minutes. Then it clicks.
There is no universal glass block you place in walls. Instead, Hytale bakes glass into specific items. That is why you can have windows without ever touching a furnace, and why smelting sand gives you a bottle rather than a building block.
So before you farm anything, decide what you actually need:

How to make glass bottles
This is the simple one.
1) Collect sand

Sand is easiest to grab from beaches, lakeshores, and deserts. If you are mining near rivers and the blocks look sandy but don’t behave like sand, you may be hitting sandstone instead. Move toward wider shorelines.
2) Smelt sand in a furnace

Make or place a furnace, add fuel, drop sand into the input slot, and run a batch. The output is empty potion bottles.
That is the whole loop. If you expected a glass block, you did nothing wrong, it just isn’t how this build handles glass.
Quick tip for faster batches
Bring more fuel than you think you need and smelt in one go. The annoying part is stopping every minute to feed the furnace, not the sand gathering.
How to craft windows
Windows are crafted, not smelted.
1) Build a Builder’s Workbench

If you do not have one yet, craft it from your standard workbench and place it in your base. It is the station for building-focused items.
2) Insert a wood log

Open the Builder’s Workbench, put in any log type, then scroll the output list until you see window options. Pick the style you like and craft.
The nice part is that different woods can give different window looks, so you can match windows to your build without hunting special materials.
Where to find sand fast
If you want sand quickly without wandering, use a simple rule: follow water until you see a real shoreline.
Good sand spots are usually:
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Beaches and coastlines
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Large lakes
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Desert edges
If you are not seeing sand at all, you are probably in a region that simply does not generate much of it. In that case, do not force it. Move toward a desert or a larger body of water and check the shoreline again.
Common mix-ups
I smelted sand and didn’t get glass blocks.
That is expected. Smelting sand produces bottles, not placeable glass.
I want windows but I’m stuck looking for sand.
You do not need sand for windows. Use the Builder’s Workbench and craft windows from wood.
My windows look different than someone else’s.
That can be as simple as using a different wood type, or playing on a build with slightly different window variants.